Hello fellow Stressers!!

My question is not related to stress analysis. It's somewhat related to Piping Material Engineering. I have received a comment regarding a reducing tee in which the material is "A234 Gr WPB, SMLS", in which I've prepared a Material Requisition. The comment is shown below:

"Check connected pipes to be API 5L Gr B or Gr X60 accordingly A234 WPBW or A860 fitting to be selected as high strength fittings in sour service are prone to SSC."

My question are the following:

1) Does A234 WPBW (Gr. B) fitting material is better than high strength fittings (Gr. X60) in terms of sour service and is not prone to SSC?

2) Is it necessary that pipes & fittings should have same grade (API 5LB pipe to A234 WPB fitting) & (API 5L X60 pipe to A860 WHPY 60 fitting) or they can be mixed? I'm wondering why we need to check the material of connected pipe, maybe this is the reason.

3) What is SSC and why the high strenght fittings (A860 WHPY 60) is prone to this and not the low or medium strenght fittings (A234 WPB)?

When I search google, A860 is high strength fittings, but in the comment it seems like it can be selected for sour service which is conflicting with the statement that high strength fittings is prone to SSC in Sour Service.

Please correct any wrong statement I've made, as I am starting to enter the world of Material Engineering & trying to read some NACE stuff.

Cheers!!!