[jitter] When do I need to put @in2_dim in jit.repos ?

I have read the jitter reference about max operator and the meaning of in, in2, in3, in2_dim, in2_type ..etc, but still do not understand it well. 

Does in, in2 or in3 mean the input on top of the object box ?

Does "dim" mean the size of the matrix coming from the right inlet ?

When do I need to put @in2_dim in jit.repos object and when not ?

Could any one explain it more to me ? Thanks.