According to Livy they deployed with the rest of the legion on the battlefield:
This body of thirty maniples they called antepilani, because behind the standards there were again stationed other fifteen companies, each of which had three sections, the first section in every company being known as pilus. The company consisted of three vexilla or “banners”; a single vexillum had sixty soldiers, two centurions, one vexillarius, or colourbearer; the company numbered a hundred and eighty-six men. The first banner led the triarii, veteran soldiers of proven valour; the second banner the rorarii, younger and less distinguished men; the third banner the accensi, who were the least dependable, and were, for that reason, assigned to the rearmost line. - History: 8.8.7-8