WUD stands for Write-up/Write-Down. It is a value or percentage at which billable hours are written up or down, thus changing the bill rate of time entries. It is the equivalent of markup for expenses. You can write up/down the value of time entries in various ways: by applying WUD at the time entry level or by changing the value of net bill while invoicing. Changing the value of WUD to 0%, resets the original value of time entries.
For example: If the original bill rate = $100 and billable hours = 5, then the bill amount is 100 x 5 = $500. If you apply a WUD of 100% to this, the new bill rate = $200 and so the new bill amount is 200 x 5 = $1000.