Add asset upon receipt works but Invoice price variance and tax line can not be capitalised

Summary:

Add asset upon receipt works but Invoice price variance and tax line can not be capitalised

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Following add asset upon receipt process found here "Using Maintenance" the flow can be enabled until point 14, in which it is stated:

You can also transfer any invoice price variance for your item and tax lines to fixed assets, when you account for your invoice in Accounts Payables. The invoice price variance is the quantity invoiced multiplied by the difference between the invoice unit price and the purchase order unit price.

This is not happening and i would like to have a step by step guide on this last process,