Each AH pair corresponds to a particular DEX trading pair, and, in the initial version of AH, each pair has to have a stablecoin as 1 of the tokens in the pair (though the team is already developing an upgrade to AH that removes this requirement, coming soon ™). Also, technically the native asset of the chain cannot be used (for simplifying the code to only tokens and reducing attack surface etc), but it also isn't used under the hood in DEXes like UniswapV2 either anyway, so it doesn't affect the user (and the user only needs to provide stablecoins anyway). For example there could be USDC-WETH and DAI-UNI pairs, but not UNI-WETH or USDC-DAI. The future upgrade to AH will enable pairs like UNI-WETH. Stablecoin-stablecoin pairs are obviously already delta-neutral, so we're not concerned with them.