By default, email notification for comments go to everyone who is "following" the feedback ticket. Following refers to users who have either voted, commented, predictive matched, or owned the feedback (Test Manager or Test Lead, for example).
The buttons let you fine tune who gets email notified. You can pick and choose, though typically, these aren't customized. Most customers find that if a user is following, then they should be notified.
Greyed out buttons will not send email notification according to their function, while ungreyed buttons will.
OS is original submitter. With that clicked, the individual who submitted the feedback gets email notified of the comment.
FO is feedback owner. The individual account who owns that ticket (clicked Check In) gets email notified of the comment.
AS is all submitters. This is everyone who matched their feedback to this ticket and if any admins found that other tickets were duplicates of this.
PO is previous owner. The last individual account who owned the ticket.
AC is all commenters. Anyone who has commented.