Two layers of limits operate at once, and confusing them causes avoidable outages.
YOUR ALLOCATION
What you bought from us: total disk, total bandwidth, total inodes, and a CPU and memory allowance shared across everything you host. Shown in Client Area > Services > My Services.
CLIENT LIMITS
What you assign to each cPanel account through packages. These divide your allocation among your clients.
HOW THEY INTERACT
A client cannot exceed their own quota. But all clients together can exhaust your allocation even while each stays within its own limit, if you allocated more in total than you hold.
When your allocation is exhausted, every account is affected regardless of individual quotas. Uploads fail, mail stops being delivered, and databases may become read-only across the board.
CPU AND MEMORY ARE DIFFERENT
These are not divided neatly. One busy or attacked client site consumes shared capacity and slows everyone. Disk quotas do not protect against this; only identifying and fixing the account does.
WHAT TO DO
Track total usage, not just per-account usage. Check WHM > List Accounts periodically and compare the sum against your plan. Act at eighty per cent, not at a hundred.