There is always a better way to tag something than using an atrociously long tag name and without using silly abbreviations which are even worse and extremely hard to find in the tag suggestions.

For your specific case, I would just drop the "implementation" part and make a defined-behavior tag. The word "implementation" doesn't appear to be of severe importance that it really needs to be a part of the tag name, and removing it cuts down on a lot of characters. Just make sure it gets a tag wiki and excerpt that clearly defines the tag's purpose.

There is always a better way to tag something than using an atrociously long tag name and without using silly abbreviations which are even worse and extremely hard to find in the tag suggestions.

For your specific case, I would just drop one of the words. Possibilities:

• defined-behavior
• implementation-behavior
• implementation-defined

Just make sure it gets a tag wiki and excerpt that clearly defines the tag's purpose.

If you're ever unsure about how a tag should be created due to the character limit, come to Meta and ask the community to help you. But increasing the character limit just isn't a good option.