When another mod tries to extract a specific fluid that is NOT water through the sink this method is called.

https://github.com/blay09/CookingForBlockheads/blob/e73972ff2ffbdeab18c2a08dbd9ec6a8fd49834f/src/main/java/net/blay09/mods/cookingforblockheads/tile/SinkTileEntity.java#L70

Normally this returns nothing. But if you insert the fluid into the sink before requesting the same fluid the sink will return water even though a different fluid was requested.

This is because when inserting into the sink the internal tank fluid gets set to that fluid.