In Part IIA you take four optional papers from the Part II papers offered by the Department of Theoretical and Applied Linguistics (Section C) and those offered by other departments (borrowed papers - Section D).
If you transfer into Part IIA of the Linguistics Tripos after completing Part I in another tripos, you will take two of the introductory papers from Part I of the Linguistics Tripos (Li1 Sounds and Words and Li2 Structures and Meanings), unless you have already taken these papers.