Text fields often hold more than one piece of information, or the same value written in different ways. String functions fix that.
SPLIT([Customer Email], "@", 2) returns the second part of the text cut at "@": the domain.LEFT(text, n), RIGHT(text, n) and MID(text, start, length) cut by position. MID counts from 1.UPPER, LOWER and PROPER change capital letters.TRIM removes spaces at both ends. A stray space makes " 00412" and "00412" two different accounts.CONTAINS, STARTSWITH and ENDSWITH return true or false.
Careful: CONTAINS and its friends are case sensitive. CONTAINS([Subject], "refund") misses "Refund request". Compare in one case: CONTAINS(LOWER([Subject]), "refund").