PERF_PREFER_INTERNAL_STITCH_JOIN is a database feature parameter.
This parameter controls where a full outer join is performed.
If it's checked, the full outer join is performed by BI Server (See stitch join) otherwise the full outer join is pushed to the back-end database (of course if the back-end database support the full outer join operation ie the FULL_OUTER_JOIN_SUPPORTED feature flag is on).
In the Administration took, Physical Layer, Properties-Feature, the feature 'PERF_PREFER_INTERNAL_STITCH_JOIN' is defined as:
PERF_PREFER_INTERNAL_STITCH_JOIN: this is used in target marker. If the flag is on, we will mark the full outer stitch
join as internal execution otherwise we will try to push full outer stitch join as full outer join to the backend.