This is the row index into the modem's stored statistics table, letting you walk through up to 255 saved samples. Because the table is capped at that size, this index is what an admin polls sequentially to pull the full sampled history of Eb/No and Tx power before the oldest entries get overwritten. An index value of 200 would point to the 200th stored statistics sample, useful for retrieving the tail end of a long monitoring run.