Use
An on-demand payroll run applies to regular pay assigned to a specific pay period. It therefore enables you to pay employees before, during, or after normal payroll accounting is performed.

You may want to run payroll in order to pay employees who have just been hired or dismissed, or who are on vacation during the regular payroll run.
Integration
On-demand payroll should be used cautiously as the normal check on the payroll control record is not made. That is, the payroll status of the employee's payroll area is not taken into account. Care must be taken to ensure that the employee is correctly accounted for not only in payroll but also in follow-up activities like data medium exchange and posting to FI/CO.
Prerequisites
To be able to use the on-demand payroll function, you must carry out the step Set up variants under On-Demand Payroll in Customizing for Payroll: United Kingdom.