Assuming SX2K or 3300:
If the key appearance is also a PRIME number on a set somewhere then it's call rerouting on first alternate will be controlled by the COS of the set where it is prime. Call reroute second alternate will always be controlled by the call rerouting timer in the system options assignment form. 2nd alternate destination is not controlled by class of service.
With no call reroute 1st or 2nd alternate, then user-programmed forwarding (if any) takes over. However, user-programmed forwarding only applies to the PRIME number. In this case the amount of ring time on each instrument in a daisy-chained string of multiple forward hops will be determined by the COS of each set in the chain. With daisy-chained user-forwarding all forward hops must be prime numbers except the final destination can be a softkey.
Black magic comes into play when we begin dealing with extensions that are not prime (anywhere) and appear only as softkey appearances.
In the first place, key appearances that are softkeys only and do not have a prime appearance anywhere cannot be user-forwarded, period. Please note the period. They can only be rerouted via the call rerouting tables.
However, CLASS OF SERVICE still comes into play. What determines the class of service of a "softkey" is the class of service assigned to the instrument where the softkey first appears. To find out, look at the MULTILINE SET GROUP (MU S G) assignment form for the softkey extension you're interested in. This form will show the PRIME EXTENSIONS of all sets where the softkey appears. The first set in the list is the one that governs class of service behavior of the softkey.