We have to consider engagement while in user Model. The 'doing' vs the projection of it, the 'knowing' vs acting of knowledge, the known vs unknown models, that gather the formulas and build upon each other. The cost, defined in hardware, vs virality, engagement metrics and retention cohorts which are also models for interaction & social metrics, whether you are social or not. Models defined in plain English which are also language models, in any language with translation as models. Archetype models, for example policeman, etc.