For the Llanwern - Shotton traffic, as I understand it, Llanwern are taking in hot-rolled coil (either from Port Talbot or imported through Newport Docks) for cold-rolling, with the cold-rolled coils then despatched to Shotton.
While on the topic of Shotton, this video is interesting:
Tata Steel Shotton Introduction
More generally on the current pattern of steel movements and different products:
Import steel heading by ship to South Wales is either in the form of slab (which has to go to Port Talbot for rolling, as the production line for rolling slab at Llanwern was mothballed years ago), or it can be in the form of steel coil (also known as strip or strip coil). Production at Llanwern is based around processing incoming coils (pickling / cold rolling / galvanising / cutting into sheets and so on). I think most of the coils coming into Llanwern would be hot-rolled; but some cold-rolled steel may also be arriving for further processing (both open and covered coil wagons are used from Port Talbot to Llanwern). An idea of operations at Llanwern is given by this Tata webpage - among other things they produce automotive steel (for body panels) and have a production line for the kind of galvanised steel barriers used at the side of roads.
https://www.tatasteeleurope.com/construction/sustainability/performance-at-our-sites/llanwern
For a general terminology of different types of steel see below - for example billet is a specific term not related to the production of steel coil / strip.
Some of this info is taken from the World Steel Association Glossary:
https://worldsteel.org/about-steel/glossary/
General gallery of different types of steel products:
https://www.wedowds.co.uk/about/gallery (N.B. the only steel moved by rail through Newport Docks is in the form of coil and slab, plus export tinplate from Trostre!)
Billet: A semi-finished steel product with a square cross-section up to 155mm x 155mm [produced at Cardiff Celsa]
Billets are used to make bars and rods (e.g. rebar / wire rod / merchant bar on site at Cardiff Celsa)
>Internal billet move for Celsa at Cardiff, with billets visible stacked 4 high and 8 wide. These will be made into wire rod at the Cardiff Rod Mill (Castle works):
https://www.flickr.com/photos/chrisperkins/49828815301/
Bloom: A semi-finished steel product with a square cross-section greater than 155mm x 155mm [produced at Scunthorpe for making "long products" i.e. structural steel, beams, rails e.t.c. ]
>Blooms on FIA wagons at Skinningrove, used for making specialist products:
https://www.flickr.com/photos/46120187@N08/52662003892/
https://britishsteel.co.uk/what-we-do/special-profiles/
>Blooms on BDA wagons heading for Skinningrove (loaded 7 wide, with 14 blooms per wagon in this case):
https://www.flickr.com/photos/32859789@N02/52608776706/
Plate: A flat-rolled product from slabs or ingots of greater thickness than sheet or strip. [manufactured at Dalzell from slab, but the plate is not moved by rail]
Sections: Structural steel products for use in construction. Beams, columns, piles e.t.c
https://britishsteel.co.uk/what-we-do/construction-steel/sections-product-range/
These are produced by British Steel at Scunthorpe and Lackenby (Teesside Beam Mill).
Not currently dispatched by rail but this is a good example of a wagon loaded with steel sections in 2008:
https://www.flickr.com/photos/richardajones/6359383205/
Sheet: A flat-rolled product over 12 inches in width and of less thickness than plate [made from cutting strip - this kind of processing is done at Llanwern among other things, but sheets are not moved by rail]
Slab: A semi-finished steel product processed through a continuous caster and cut into various lengths. The slab has a rectangular cross section and is used as a starting material in the production process of flat products, i.e. hot rolled coils or plates. [produced at Port Talbot and Scunthorpe, also imported through Cardiff / Newport / Portbury docks for rolling into strip at Port Talbot]
>Chunky slabs seen loaded to BTA wagons, although larger slabs (in some cases weighing 25 tons) can only have 2 loaded to a BTA due to the weight:
https://www.flickr.com/photos/nat37670/53126085150/
Tata slab traffic is now mostly loaded on BYAs converted from coil carriers (3 slabs per wagon is normal):
https://www.flickr.com/photos/chrisperkins/53718127375/
The only Tata facility currently processing slab is Port Talbot, so all slab traffic from Newport Docks / Cardiff Docks / Portbury is heading to port Talbot for hot-rolling into strip (i.e "hot rolled coil").
There was a production line at Llanwern for rolling slabs, but this was mothballed years ago and production at Llanwern now involves processing coils from Port Talbot (or imported through Newport Docks).
Strip: Flat steel coil products, with widths of less than 600mm for hot rolled products and less than 500mm for cold-rolled products. The wider flat products are called wide strips.
>"Strip" is firstly hot-rolled from slab (hot-rolled coil) at Port Talbot (or imported). Locations using hot-rolled coil are Trostre tinplate works, Corby tube works, Hartlepool pipe works, and Llanwern.
Hot-rolled coil can then be pickled (surface scaling removed) and cold-rolled (reduced in thickness and turned into a product suitable for use in manufacturing, e.g. for car body panels or domestic appliances).
Hot-rolled coils are the big coils transported in open wagons, often while still hot (can sometimes be seen steaming in the rain!) while cold-rolled coils need to be transported in covered wagons for protection from rain.
Wire Rod: Coiled steel rod which has a number of end uses and applications, including tie wire, mesh, fencing, shelving, shopping carts, wire hangers and nails. A typical use of wire rod from Cardiff Celsa would be in the production of reinforcing mesh, where the wire is welded into a grid to strengthen poured concrete used on constructions projects. [despatched from Cardiff Celsa to Burton in VGA vans]