the rach ta filter is a value (distance, in steps of 500m) will prevent the mobiles located further away than a certain distance from the BTS.
By default it is defined to 63 (max. distance), all mobile within a radius of 32km will be accepted in the cell. Accepted = they can camp in idle mode, receive and send calls, etc.
You can decide to reduce the distance in certain cases, if the cell is resurging, or if it creates somehow a conflict with other cells. Well, many cases are available. For instance, you might want to limit the rach ta filter to 1 for micro cells, so they don’t catch mobiles that are located more than 500m away.