To record a stand down against a hire:

Open a new or existing Invoice.
From the Options dropdown link just above the invoice items grid (below the Hired From/To dates), choose the Stand Downs option.
The Stand Downs List will appear, which shows any stand downs already recorded on this invoice.
Click the +New Stand Down button to add a new stand down record.
Check the box to the left of the invoice line items you wish to stand down.
Enter a Start Date.
If the finish date is unknown, you can leave the Finish Date empty and enter this after the stand down is actually completed.
Enter reason for stand down and any notes towards the bottom.
Click Save New to save the stand down record.
Close the Stand Down List to return to the Invoice page.

If you wish to recalculate the Invoice Qty/Units based on the stand down deduction, you with need to run Autocalc.

Stand Downs (Overview)