Fxd Resets Before

Key Fxd Resets Before in Advanced Pricing refers to an optional date that sets the terminal date Tᵣ by which all index fixings are read from the supplied historical data, rather than implied by the available curves.
Concretely, historical data are used only when a fixing date occurs strictly prior to Tᵣ
If not specified, it is regarded equal to the given spot date.

By default, the spot date equals the globally set trade date ( i.e. valuation date) that usually equals today's date.
But if the As Of entry to the function Price exists, the latter date is then regarded as the spot date.
It is represented by the time 0 in the documentation context of Advanced Pricing

Setting it to a date greater than the given spot date (denoted as T₀) becomes useful when one is interested in the PV or the cash flows under the condition that all index fixings have attained pre-defined values in the time interval until the specified date Tᵣ

Note, an error will be issued if the index fixings dated prior to the date here are not supplied as part of an input object of type Historical Values in the course of a valuation.