You can add comments to a document as long as you have one or more Workflow tasks assigned to you for that document. Document-level comments are used to record your feedback regarding one specific document or to clarify a markup you have made.
There are two types of comments: user comments and standard comments. User comments are custom comments created by the user to address a specific issue. Standard comments are pre-defined comments that are created by a system administrator. Standard comments can provide a convenient way to quickly add often-used comments to documents.
Note:
Depending on the configuration of your system, standard comments may not be available.