SEC wire capacity just short of main circuit breaker size.

[Mike Lamb 66]

Is it worth noting – and/or recommending action – when the SEC has an amp capacity of 90 with a 100 amp main disconnect? In this case, #2 AL, THW.

[Jim Katen 166]

If this is a service entrance conductor that's carrying the entire load of the service and if there are no derating issues at play, then you only need to size the wire at 83% of the load. In effect, a 100-amp service only needs to have 83-amp service entrance wires. That makes #2 AL THW a perfectly valid choice. 

[Jim Katen 166]

Or just use this handy pocket table: 

AWG table.pdf

[Marc 90]

Table 310.15(B)(6) of the NEC specifies #2 aluminum as adequate for a 100 amp service in residential.

[Jim Katen 166]

Depends on the edition of the NEC. That table was removed in the 2014 & 2017 NEC editions and replaced with the 83% rule. Then the 2020 edition brought it back, but it's now Table 310.12 and it carries the caveat that it can only be used when there are no adjustment or correction factors required. It's exactly the same as the document that I attached.