You can write comments for all entities in ATLAS.ti. Comments -- different from memos -- are always directly connected to the entity you write them for. Memos are independent stand-alone entities that have a name and a type. Memos can be grouped and can also have comments on their own.
Writing comments is similar to scribbling notes in the margin of a paper, or attaching sticky notes to things. Comments can be written for documents, quotations, codes, memos, networks, all types of groups, and for relations.
See Working with Memos and Comments for further detail.