There are few special fields also appear in the mappings selection, which are the Comment and Comment Format fields. See highlighted here:

If a data is mapped to the Comment or Comment2 field that will be added as a new comment to the issue being created from that row! This is a very convenient way to add comments to issues.

The Comment Format or Comment2 Format field is specifying the format of the "Comment"/"Comment2" being added.

The possible values are "W" for "Wiki", "H" for HTML, and "P" for plain text. If the Comment Format is not specified the comment is added in plain text format.