Schema Lock: Add Field vs. Editing

Hey all,

I thought I understood this concept better than I do, but apparently not. I need a refresher course.

I understand the basics of schema locks, however I'm having trouble with something simple.

I've got a couple feature classes in a file geodatabase (ArcGIS for Desktop Basic 10.1, simple feature classes, nothing complex).

My colleague has several mxd's open which have these feature classes as layers. I'd appreciate it if someone could provide me a detailed explanation of why I'm able to edit these datasets (add/delete data, etc...), but unable to Add a field to the attribute table. I didn't get the explanation I was looking for from the help files.

I was under the impression that schema locks for read access and locks for editing were different. I guess I thought that, if someone is editing that feature class, it's locked fully, but if they're simply accessing it, I could perform things like Add field or edit.

Long story short: what are the specific differences of "Add Field" and Editing a feature class in terms of locks. A less important, but if you have time question: do shapefiles share the same type of locks?

I hope that makes sense, and thanks for your time!