Job Offers Respect Headcount of Job Requisitions and Positions

As candidates move through the lifecycle, validations are done to check if the business still needs an additional worker or whether the opportunity is no longer available. This includes checking whether the requisition still has openings and, for position-based offers, whether the position still has enough open headcounts and vacant FTE as of the offer's intended start date.

Your administrator can configure the application to fully control how to manage requisitions and positions to ensure offers don't exceed various limits.

Communicate Job Offers Ignoring Requisition Limits: This requisition-related privilege allows Recruiting users to hire more candidates than the number of openings on the requisition.
Number of Incumbents Validation: This position-related setting controls whether all positions' headcount is enforced across your organization. When the setting is disabled, no verification is done to check if there are still open headcounts and vacant FTE in any position.
Allow Overlap: This option is available for each position to control whether extra people can be hired when that position's headcount and FTE limit have been reached.

When are these limits considered to be reached?

A position considers a candidate to be an incumbent as of their proposed start date, as soon as their job application has reached the status Offer - Extended in the recruiting lifecycle (and all later states, except the states Withdrawn by Candidate or Rejected by Employer in any phase).
A requisition increases its count of hired people as soon as a candidate reaches the HR phase, which is the final phase of the recruiting lifecycle (unless they're in the states Withdrawn by Candidate or Rejected by Employer, or Pending Automated Processing in that phase).

When Are Verifications Done

Here's when validations are performed in the job application's lifecycle:

When using the Submit button: A job offer can be drafted for a candidate's job application regardless of any limits, but when the user is ready to submit it into the approval cycle, these validations occur. When clicking the Submit button in the flow Create Offer or Edit Offer, the user might receive a warning or error message. This saves the approvers' time, because the job offer can't be submitted if the requisition doesn't have any remaining openings - unless the user has the privilege to ignore this limit. The draft offer also can't be submitted if it has a position which doesn't have enough open headcount or enough vacant FTE (unless the Number of Incumbents Validation is disabled). However, if the position is configured to allow overlap, the user is warned that this candidate will be overfilling the position as of their offer start date, but they're given the choice of continuing anyway and submitting the draft offer for approval.
When using the Extend Offer action: By the time the user is ready to extend the draft job offer to the candidate, it's possible that the requisition or position was updated or became filled, so that no room now remains for this candidate. The same validations are performed before the status can be moved to Offer - Extended, displaying the appropriate error or warning based on the requisition and any position according to the relevant privilege, setting, and option above.
When the automatic action Extend Offer is triggered in the candidate selection process: If the requisition's candidate selection process is configured to extend job offers, either immediately or after a delay, these validations occur. However, as there's no Recruiting user to consider any warnings, only errors can prevent the action and leave the job application in the status Offer - Approved. The requisition's count of hired candidates isn't checked by default, but the automatic progression can be configured to prevent the extend of the offer if the requisition is full. For position-based offers, the position's number of incumbents is checked, and the automatic action is successful if the position has room or if it allows overlap. If there's no room, and the position doesn't allow overlap, the candidate's job application remains in the status Offer - Approved.
Never During Accept: As long as the job offer is in the state Extended, the candidate isn't prevented from accepting the offer. This is also true for the Recruiting user who might accept the offer on behalf of the candidate. Even if the requisition or any position was updated or became filled in the meantime, the candidate and users aren't informed of these limits. It will be the responsibility of the HR specialists to reconcile any problems after this point.
During Move to HR, using the menu action or the automatic candidate selection process action: When the candidate moves into the HR phase, no further validation is done for the requisition's limits. It's still important to not overfill a position's headcount and FTE, so these validations do occur if the offer has a position. The move into the HR phase is always successful. However, the Recruiting user needs the HR specialist to reconcile any issues that arose from position changes in the meantime. If the position will already have enough headcount or FTE as of the offer's start date, the candidate will be moved into the HR phase but in the state Error During Processing. If the position does allow overlap, the candidate will be moved into the HR phase without a problem, although the warning will be visible in the Errors section of the Offer page. If the move to HR is done automatically using a process (not manually), the warning message alert will be sent to the hiring manager.