To check the resource usage of the domains hosted under your account, please follow the following steps :-
1) Login to Plesk control panel.
2) Click on "Tools & Utilities".
3) Click on "Resource Usage By Domains" and it will show the usuage of resources
for all the domains hosted under your account.
4) Following is the details of the resources showed :-
DU :- Disk Usage
TR :-Traffic Usuage
SD :- Subdomain
WU :- Web users
DB :-Database
Mbox :- Mail boxes
MA :- Mail Aliases
MR :- Mail redirection
MG :- Mail groups
ML :- Mailing Lists