I think the sheets department were in the yard to the north of Shrub Hill, in the triangle of lines there. It was a significant part of railway inventory for large canvas sheets, sometimes known as tarpaulins, being used to cover goods in open wagons. There were many thousands of them, serial numbered, on a large railway like the old GWR, and they needed distribution and regular repair, being sent back and forth to this central point. For lorries nowadays, covering loads with a large tarpaulin is still known as "sheeting" the goods. They would be sent back and forth between the central stores and the various goods depots by the wagonload, of course. They were a significant capital investment for the railway. If they went onto a "foreign" line, not only the wagon but the sheet as well had to be returned, the serial numbers having been taken at the exchange point.
The Midland Railway had the same, the name still being used for Sheet Stores Junction, on the line between Derby and Nottingham.