First, I added the injection tag to Should IP addresses be validated to prevent SQL Injection?.
Well tagging question, except when the scope of the tag is really obvious, I usually consult the help text of the tag:
About injection
inserting malicious content, usually code (like SQL, Javascript)
into a vulnerable application
In this case the help text explicitly makes the tag appropriate for both SQL injection and XSS.
(When the help text is too vague, unclear, or even missing, I see which other questions use the tag and try to stay consistent with existing usage.)