A separate Ad-ID code is required per unique advertising asset.
For example, if the same ad is being created with different offers at the end, each version should have its own unique code. If any revisions are made to the ad, such as an image change, a URL shown on the screen changes, or 3 seconds of audio change, a new Ad-ID code should be created.
In situations where the same exact ad is being sent in various lengths or sizes (such as a 15 second and 30 second version of a video, or a half page and full page version of a print ad), the best practice is to create separate codes for each length/size even if the creative is the same.
If the same exact ad is being sent to multiple publishers (TV network, online publisher, etc.), the same Ad-ID code would be used since there is no variation in the ad from publisher to publisher. There are cases, however, where an agency/advertiser sends the same ad to multiple publishers, but the publisher then adds different tags, music, etc. Even though there are variations being done to the ad, since the agency/advertiser is sending the same version out to everyone, they would create one code.
If creating codes for an ad that has different aspect ratios (19:9, 1:1, 4:5), the best practice would be to create one code. This is because there are no actual changes in the content or size of the ad.