Consider a futures contract on BTC which is quoted in USD. Here, BTC is the base currency and USD is the quote currency.

In a typical (aka vanilla) futures contract, the margin and profit/ loss are denominated in the quote currency. Thus, a vanila futures on BTC that is quoted in USD is margined and settled in USD.

However, in the case of an inverse contract, margin and profit/ loss are denominated in the base currency. Thus, an inverse futures on BTC that is quoted in USD is margined and settled in BTC.