Custom messages are used to change the default phrasing within the project.
For example, I may wish for my 'Quote Requests' project to use the term 'quote request' instead of 'ticket'.
Custom messages can be implemented from Admin > Infocapture > Your Project > Custom Messages

These will appear in the corresponding places within the project:

Reminder: If your changes do not appear straight away, delete the site cache and check again.