Tax charged on your sales, month by month and sale by sale — the figures you need for your VAT return.
For filing: each calendar month in the period (part months are clipped to your dates).
How this is worked out. Tax is the tax recorded on each sale. “Sales before tax” is after discounts. A sale synced twice counts once and sales on hold are left out, the same as the Sales report. The tax rate shown per sale is tax ÷ sales before tax.