A tag ID is an identifier that you put on your page to load a given Google tag. Examples of tag IDs include GT-XXXXXXXXX, G-XXXXXXXXX and AW-XXXXXXXXX. A single Google tag can have multiple tag IDs.
Instead of managing multiple tags for different Google product accounts, you can use the Google tag across your entire website and connect the tag to multiple destinations.
When you're in the Google tag section of your Google Ads or Google Analytics account, your tag ID is displayed under the Google tag on the left side. A single tag can have multiple tag IDs. Sometimes multiple tag IDs are displayed after combining tags. The first ID is the 'main' ID, but the IDs can be used interchangeably when adding the Google tag to your website. If you remove a destination, it doesn’t change existing tag IDs.