Differences between Comments and Communications

I use ERPNext from a few days and I admit that I don’t understand the difference between comments and communications.
If I create a comment, the customer does not receive it? If I create a communication, is it an email? For an another example, when I create a comment on an Issue, Customer cannot view it on him Issue page on website. Why Comments and Communications are not the same finally ?
Could you enlighten me on the difference between these two features?
Besides, how is the “timeline” on each doctype constructed? Is there another DocType that allows you to display the activity list on the timeline?
Thank you for your help.


A list of all Communication with a Contact or Lead. All emails sent from the system are added to the Communication table.

If you see on each and every doc type comment section are there. Once it save it get added in to Comment master list and vice versa. On Comment master ,if any comment gets mark and linked with any doc type ,user can view that comment under resp doc