Begin Date

This field identifies a begin date for a particular user assignment.  This is a required field.

The user is allowed to add and save hours. However, if the user adds hours to their timesheet (INUSE status) that fall outside of the specified begin and end dates, the user is prevented from submitting their timesheet.
A validation error is displayed stating that the timesheet cannot be submitted until corrections are made.

Users cannot have two assignments with overlapping date ranges (on the same project or task).

When using the Assign with Grid option, the begin and end dates correspond to the Assigning Periods criteria selected on the Search tab.

Note that if the Link Dates to Project / Task Dates option is checked, the Assignment dates are derived from the project (or task) dates (depending on whether the assignments are managed at the project or task level for this project) and cannot be directly entered on an Assignment.

Note about budget of hours: This allows you to specify an estimated level of effort for the user which may be capped if desired. This is done by leaving the option to Allow Budget to be Exceeded unchecked.
The user is allowed to add and save hours. However, if the user exceeds the budget in their assignment, the user is prevented from submitting their timesheet. A validation error is displayed stating that the timesheet cannot be submitted until corrections are made.