Thanks for the replies.
It would be somewhat important to know if the SCCR at the branch panels depends on what we are starting with at the main feeder breaker.
On the other hand the SCCA at the main breaker may have to be significantly lower to make a difference at the branch. I did some rough quick comparison calcs and, for example, starting with 100K at the service, the SCCA at a branch was greater than 10KA. And using a lower SCCA at the main circuit breaker still resulted in >10KA. I think for my example, the only difference would be the SCCR of the main distribution panel (65KA vs 100KA or what ever).
SCCR will be less at your branch panels, how much will depend on size and length of conductor to the feeder breaker. With 200 amp and less conductor just going from 10 feet of conductor to 25 feet usually makes a huge difference. Don't forget if you have 100k at service, then go to a distribution panel before going to a branch panel, you drop some just between the service and distribution panel.
100k at service is pretty high - you must have transformer very close to service I'd guess to get that high, especially if it is only 600 amp supply. I just plugged in (well oversized for 600 amps @ 208/120) 500 kVA transformer into fault current calculator and pretty unlikely low impedance (just to make the result high) of 2% and still only come up with about 69 kA of fault current at transformer terminals. Then 10 feet of parallel 350's to service disconnect drops it to 61.5kA, 25 feet drops it to 52.6 kA, and 50 feet drops it to 42.4 kA.
If I plug in values that seem more likely for a 600 amp service - 225 kVA transformer and maybe 4.0% impedance (might be higher than that, so this will yield a high fault current) I only get 15.6kA available at transformer terminals.
Other possibility is someone gave you a worst case scenario of 100kA, but you likely aren't even close to that.