You cannot use purposed this way. Purposed is the passive participle of the (now very rare) verb purpose, meaning "have as one's purpose": it designates your goal or object, not something on which you intend to operate in order to achieve your purpose.
We have in fact no attributive adjective or participle with the generic sense you intend. In any case, your best course is to employ whatever verb most precisely designates the operation you are carrying out; it is almost always better to be as precise as possible than to employ vague generics like involved or targeted. If what you're doing with that section is "parapositing" it (don't look that up, I just invented it), say so:
... encloses the paraposited section.
... encloses the section being paraposited.
... encloses the section under paraposition.
Your second-best course is to follow AmD's suggestion and employ a following participle or preposition phrase.
... encloses the section involved.
... encloses the section being operated upon.
... encloses the section in play.
... encloses the section under consideration.