This will relate to any attachments that have been added to the return.

If you are filing an SA100, SA800 or SA900 you can have up to five PDF attachments, but the total size of all attachments must be less than 1mb.

If you are filing a CT600 you can file with one set of iXBRL accounts and computation and then attach one further PDF file under the heading of other. This will need to again be less than 1mb.