Tier levels are based primarily on what they call "client loyalty", which is determined by whether an athlete has had one or multiple sessions with a coach. This status can change each month and is based on percentages. Every coach starts at the silver level and can move up or down. However, no coach can be gold unless they have completed over 100 client sessions total. Basically, an experienced coach with which clients complete multiple sessions is at a higher tier than someone who is frequently "one and done" for whatever reason with clients. Client reviews can also give you another measure of how well a coach is received, but these are often done after a first session, so the tier level adds a little more depth to that measure.
100+ Completed Sessions
50% - 100% Client Loyalty Rate
25% - 49% Client Loyalty Rate or better, but less than 100 sessions completed