A (2) on the thread would be the ply— two strands twisted together - some threads are (3) strands. You can have (2) ply very heavy thread. A (2) for weight would be very thick- perle cotton flosses are generally 5 ( very thick, 8 & 12 weight) sewing threads are usually 40, 50, 60, 80 and 100 weight. 100 weight is usually silk thread— very very fine/ thin thread. 40 & 50 is the most common used for Piecing and quilting.