This seems like a pretty easy thing to find in the manual -- what did your search find?
I could not understand the PARMs inside the RA clause... it seems it trying to convery NULLS to SPACES but what 37 and 0 tells is quite cryptic for me.
Generally 37,1 means only one byte that is 37th byte.But 37,0 i could not understand.
This FIle-Aid statement does copy the records
1). IF 23rd column of input record has '1' while replacing all the low-values with SPACES starting from 37th column till end of record.
2). Also, copies records when 23rd column is NOT equal to '1' while replacing 15 bytes starting from 819 column with SPACES.