Comprehensive Detailed Explanation with all NCARB ARE 5.0 Project Development and Documentation (PDD) Study Guide Reference:

In wood frame construction, blocking installed between joists at regular intervals (commonly 48 inches on center) serves primarily to brace and stabilize the joists laterally, preventing twisting and lateral displacement under load.

The 2x4 blocking at point X, placed perpendicular between the ceiling joists, acts as cross bracing.

It resists lateral torsional buckling of the joists and distributes loads evenly.

It also helps maintain alignment during construction and after the finish materials are installed.

The blocking does not support the partition top plate directly (that is the function of the studs beneath), nor does it transfer lateral load to the 1x8. Its main role is structural bracing for the joists.

Supporting Reference:

NCARB ARE 5.0 Review Manual, Project Development and Documentation, Structural Systems chapter

Wood Frame Construction details from International Residential Code (IRC)

Building construction texts such as ''Fundamentals of Building Construction'' by Allen and Iano, which describe blocking used to brace joists.