In the optional Discussion status column of a list (indicated by the 
icon), objects that have discussion comments are distinguished by one of these discussion icons:
means you have read all comments in the discussion.
means there are some comments in the discussion you have not read.
To see a discussion, with or without comments (for example, to add a comment), either click on a discussion icon, or select a single object, and pick View > Discussion.
To sort a list of objects according to their discussion comments (read, unread, or none), click 
at the top of the Discussion status column. You can turn off the Discussion status column by using Display Setting preferences for columns.
You can mark discussions as having all read or unread comments. For example, if you want a visual reminder when only new comments are added to a particular discussion, select or open the object it is associated with, and pick File > Mark Discussion Read. Conversely, you can make all comments appear to be unread with File > Mark Discussion Unread. Selecting multiple objects applies these commands to each object in the selection.