display suite: limit field - determine what bundle values?
It's the machine name. You can think of the bundle as the specific settings for a specific entity. So, DS is asking you for the machine name associated with the individual content type, vocabulary, etc. that you'd like to target.

For example, say you've created node configurations (Structure > Content Types) called "Article," "Store Location," and "Job Openings." If you'd like to make your DS custom code only available in the display settings for "Job Openings," you'd put in...

job_openings|*


...or you could limit it to a specific view mode by specifying which one in place of that wildcard, such as...

job_openings|teaser


Easiest way to find the machine name is to look at the edit url for your entity and/or view mode.