Not sure if there is actually a difference between sub-sonic rimfire ammo and standard velocity ammo or just a marketing ploy.

Was looking to purchase some Eley Match and Target ammo. Velocity is rated at 1085 fps at the muzzle. Their sub-sonic is rated at 1065 fps as the muzzle. Sound barrier (temperature, humidity dependent) is generally around 1100 fps.

This just something peculiar to Eley given their match/target niche?

CCI Standard Velocity stuff is around this range, but no doubt their Mini-mag and Stinger ammo is super sonic.