Minecraft wiki entry for tag, associated text: Specifies the name of the tag to be added into or removed from the targets.
Must be a string. In Java Edition, it must be in a single word (Allowed characters include: -, +, ., _, A-Z, a-z, and 0-9). In Bedrock Edition, it must be a single word (no spaces) or a quoted string.
In Java Edition, valid characters are uppercase and lowercase letters, numbers, hyphen, underscore, plus sign and period. In Bedrock Edition, it can include any characters.