It may work like this. Coach gets the 3 swimmers highest on her list through ED commitments. Then needs a few more swimmers who apply either ED or RD. There are likely a lot of kids (many more than the 3 other swimmers she needs) the coach has been talking with who she would love to have on the team. Coach doesn’t have a lot of sway with admissions at this point, though the applications are likely flagged as potential athletes. Coach may or may not have favorites in this group. Then the coach waits to see who gets in. Terminology may vary by school, but some call it a preferred walk on.