In this question, specifically, in the force diagram, why we don't care about the Fba(compression) and Fbc(tension)??

Is there a reason that defines internal forces as Fba(tension) and Fbc(compression)

2m Determine all the internal forces of the truss by the Method of Joints. B A pinned support 500 N 45 2 m C A roller support Solution The free body diagram of Joint B is shown below. B -45 500 N FBC (compression) FBA(tension) Applying ZFH = 0, then + 500 - FBC sin 45 = 0 FBC= 500 / sin 45 = 707.1 N