Function allocation is the design decision which assigns work functions to all agents in a team, both human and automated. This paper reviews issues with function allocation to identify important requirements to model human-automation function allocation. Of note, many issues may only be observed within the detailed dynamics of simulation or actual operations, particularly when a function allocation requires tightly-coupled interactions. This paper develops models of function allocation that extend beyond established methods to increase the number of concerns that can be examined simultaneously through both static analysis and dynamic computational simulations. An exemplar, function allocation in a modern air transport flightdeck, is then detailed, highlighting the range of phenomenon these models can describe. A follow-on companion paper discusses specific metrics of function allocation that can be derived from this model as well as from observations in high-fidelity human-in-the-loop simulations or real operations.