rpcgen tries to generate more efficient code by using xdr_inline() when possible (see the xdr_admin(3NSL) man page). When a structure contains elements that xdr_inline() can be used on (for example integer(), long(), bool()), the relevant portion of the structure is packed with xdr_inline(). A default of five or more packed elements in sequence causes inline code to be generated. You can change this default with the -i flag. The rpcgen -i 3 test.x command causes rpcgen to start generating inline code after three qualifying elements are found in sequence. The rpcgen -i 0 test.x command prevents any inline code from being generated.
In most situations, you do not need to use the -i flag. The _xdr.c stub is the only file affected by this feature.