Deribit has 4 types of symbols: Index, Perpetual Futures, Futures, and Options.
Index: Coin name, either "BTC" or "ETH".
Perpetual Futures: [Coin]-PERPETUAL, e.g. "ETH-PERPETUAL".
Futures: [Coin]-DDMMMYY, where MMM is a 3-letter capitalized string, e.g. "BTC-26MAR21".
Options: [Coin]-DDMMMYY-[Strike]-[P/C], e.g. "ETH-25JUN21-340-C".
For options and futures, the contract functions can be used with "BTC" or "ETH" as the underlying.