SWITCH compares one value with a list and returns the result next to the first match:
=SWITCH(B2,"S","Small","M","Medium","L","Large","Unknown")
It reads like a lookup table written into the formula: value, result, value, result... and an optional last argument, the default, used when nothing matches. Leave the default out and an unmatched value gives #N/A.
SWITCH only checks for exact matches. For ranges like "90 and above", use IFS. For long lists that change, keep a real table and use XLOOKUP instead, so you don't edit formulas.