SKU Must Match Pattern and SKU Must NOT Match Pattern work for when you are trying to target several SKUs at a time, using Regular Expressions. For more information about regular expressions, we suggest viewing this Regular Expression Cheat Sheet.
For example, let's say you have several SKUs that contain a pattern, such as ABC123, ABC456, ABC789, ABCXXX, etc. Instead of individually listing each SKU in the "SKU Is" field, you can list them as ^ABC in the "SKU Must Match Pattern" field and your message will send to every order that contains that ABC SKU sequence. The symbol ^ is a regular expression and it just means "starts with."
To include multiple SKU patterns in this field, separate them by using a pipe symbol "|" (On most U.S. keyboards, the pipe key is above the Enter or Return key and can be created by holding the "Shift" and the "Pipe Key" keys). Please note, you don't need include a pipe key after the final SKU pattern.
This same principle applies to the SKU Pattern Must NOT Match Pattern field, and would work as an alternative to the "SKU Is Not" field.