Intuitively Relational.
A unique relation system keeps you in charge of how your Entities and Objects relate each other. Queries feed data straight to your code, filtered, and parallel across CPU cores.
In a tiny nutshell, fennecs is an archetype-based ECS that proves: "small", "clever", and "fast" don't have to be mutually exclusive attributes to look for in a game or simulation development library. And, by the way, did we mention it's free & libre open source software (MIT license)?