Public comments are comments made by the public

or non-governmental agencies, regarding your CRA status. WHat does this mean? If a local small business entity writes a letter to the bank, complaining that the banks is ignoring the small business needs of it's assessment area (or community) and thereby not abiding by the spirit of CRA, you'd have a public comment to file. You then want to respond and file that response with the comment.
http://www.suntrust.com/common/CRA/comments.asp This is a public file section from a large bank looking at merger. Comments on their CRA activities are posted here.
What you wouldn't put in your file would be advertisements, thank you letters, donation requests (unless they envoked Sunshine) etc. General letters from agencies or people that thank the bank for it's assistance (IE FNB is the best bank in our community. We want to thank them for offering the Girl Scouts a place to hold our last council meeting....blah blah). The letter is nice but it's not addressing CRA or your service efforts in a specific manner.
Don't clog your CRA file with general comments and donation requests. I've seen it time and time again. Keep it to specific comments about your services/lending to the communities you serve. And again, if a request envokes Sunshine, be sure to keep that in both the main file for public comments and in a special folder for Sunshine agreements.