The two extremes are pure FIFO, where the order that arrived first is filled first, and pure pro rata, where every resting order at the price gets a share proportional to its size. Most CME products use FIFO; many rate products use hybrid algorithms that give a top-of-book priority to the first order and split the rest pro rata.
The algorithm shapes behaviour. Under FIFO, being early matters and queue position is an asset worth protecting, so traders avoid cancelling and re-entering. Under pro-rata-matching, size matters, so participants quote far more than they want filled.
Example: 500 lots rest at a price, of which yours is 50 entered fourth. A 100-lot buyer under FIFO fills the first three orders entirely and you may get nothing. Under pro rata you receive roughly 50/500 x 100 = 10 lots.
Related: pro-rata-matching, globex, order-book, implied-orders, price-banding