Currently, there is no email notification to requestor and approvers when a comment is made in a Purchase Request.
During the approval process, there are always some back-and-forth discussions about the request.
Without email notification, it is not possible for the users to rely on this comment box for discussion. Users are not checking / using Zoho Expense on daily basis and would not check the system frequently.
Zoho should enable the notification for comments made, just like Report Module.