When you provide liquidity in a price range that includes the current price (e.g. if the current price is 1,000 and the selected range is 500 — 2,000) then both assets are required.
However, if you wish to select a price range that does not include the current market price, then just a single asset is required. For example, if the current market price is 1,000 and your select a range of 1,500 — 2,000, then you will provide liquidity in a single token. Please note you will not earn any fees while the market price is outside of your LP position price range. This type of strategy is often referred to as a Range Order because your position will automatically convert from Token A to Token B as the price increases from 1,500 to 2,000.