Hi Muchan,
I agree The Forum has created a great POW resource.
Frederick originally enlisted in 20th Bttn (probably 12/1914)and arrived overseas after 31/12/1915. He was posted to 26th (Training Reserve) Bttn, which was Home based - possibly wounded or sick. As the 20th Bttn had gone to France in Nov 1915, it's also possible he was then transferred to 26th Bttn, although that would not be consistent with his medal roll showing overseas service with 20th Bttn. He then went to / returned to France where he was posted to 17th Bttn. Sorry if this is confusing.
Cheers and welcome.
Tim