this point is better explained in the Official Errata & FAQ file, that you can find in the Files section here on BGG:
Q: Do the limitations to Requisition Action refer to both the acting and the target Project? Also, can you summarize these limitations step by step?
A: Except for the Missions eligible only as targets, the limitations refer to both the acting and the target project.
- Not Approved Projects* can be selected.
- Approved non-Mission Projects can be selected only if they have a Plant chit deployed on the map.
- Approved Mission Projects can be selected*.
- Completed but not yet Finished Projects* can be selected (if not a Mission, they usually still have a Plant on the map because yet not Finished).
- Finished Projects can never be selected.
- Projects in Event Cards can never be selected.
*: Mission Projects can be selected only as targets.