Implementation model or system model is a representation of how a machine or a program actually works, It can be a scheme, description, pseudo code, etc.

Mental model or conceptual model is our explanation of how things work, mental models can be the same as the implementation model, if it is easier for us to imagine all the actions the way they actually performed. It is not always possible and needed, mental model is an explanation which is sufficient for us to comfortably use a tool.

In fig (1), the way engineers must build software is often a given, dictated by various technical and business constraints, The model for how the software actually works is called the implementation model, The way users perceive the jobs they need to do and how the program helps them do it is their mental model of interaction with the software, it is based on their own ideas of how they do their jobs and how computers might work.

User interfaces should be based on user mental models rather than implementation models.

Goal directed interactions reflect user mental models. The mental model look like the way the user thinks the machine or software works. It represents information for the human mind as opposed to a computer information is aggregated into meaningful concepts such as visuals, graphs, colors, metaphors and buttons.

User interfaces designed by engineers follow the implementation model.