The ticket is valid on all routes between Burnham and the boundary stations of Zone 6 (around 20 different stations in total).
In most cases, this route will be into Paddington and cross London, but in some cases alternative routes are available.
The most obvious candidates are Feltham and Surbiton.
Burnham - Feltham is priced £17.20 SDS Any Permitted, £13.60 CDS Any Permitted. Burnham has routeing points Slough + Reading.
Reading - Feltham is priced £12 SDS Any Permitted, and Slough has the same fares as Burnham.
So both RPs are valid, and hence you can use mapped routes Reading - Feltham and also Slough - Feltham.
Reading - Feltham is obviously valid via Wokingham, so you can travel Burnham - Reading - Feltham. Slough - Feltham is also mapped via Reading, so that's double proof, if you will, that you can travel Burnham - Reading - Feltham (or Waterloo)
For Surbiton the fares are
Burnham £15.10/£13.60
Slough ditto
Reading £26.30/£21.80
so Slough is a valid routeing point, but Reading is not.
Slough - Surbiton is mapped LONDON, which means that you can't go 'round the outside', and hence the ticket has no extra validity.
Burnham - Ewell East/West is £15.60/£13.40
as is Slough
Reading is more expensive
Ewell East/West are associated with Epsom and Sutton
Slough - Epsom/Sutton is routed LONDON, so again no good here.
So that's probably it, but you can check the other Boundary Zone 6 stations if you like: you can travel via Windsor, you can travel via Reading and Feltham, and you can travel via West Drayton to Paddington.
At least in theory.
In practice on any other route other than into Paddington, you may face 'real world difficulties'.