Bear in mind what I said about the OUTREC.
And compare to this:
BUILD was introduced for clarity.
FIELDS= is "overloaded" (means different things in different contexts, see SORT, MERGE, SUM, JOINKEYS, REFORMAT, for instance).
OUTREC is overloaded. OUTREC is a control statement, and also a parameter of OUTFIL.
BUILD is only BUILD.
BUILD is an "alias" of FIELDS on INREC and OUTREC and OUTREC on OUTFIL. Function is identical, and SORT does not mind at all whether you use FIELDS on INREC and OUTREC and OUTREC on OUTFIL, but for the human reader, it is much clearer to use BUILD for any new code.
Existing code can be changed, but ensure that you have the OK to do that in your specification. It is entirely unproblematic to make the change (because it is just an alias) but if you accidentally break something else at the time, you won't be popular.