Model relationship graph
A node is a model you can calculate with; an edge is a reason to switch. Every edge carries the dimensionless parameter and threshold that decide the move. This graph, the model map and the picker read the same data files.
How to read this graph
A node is not a theory; it is a model you can calculate with. “Classical particle” is one model, “classical particle with drag” is another: same laws, one extra force, a different range of validity. Splitting them makes “when do I add that term” a question you can answer on the graph.
Four kinds of edge, four reasons to switch. Limit (a small parameter goes to zero and the richer model collapses to the simpler one), correction (keep the description, add an effect that exceeds the error budget), change of description (swap the objects being represented), coupling (two sets of laws must both be present).
Every edge carries a criterion and a threshold. An edge without a criterion is merely “related”, not “switch”. Relativity reduces to Newton with v/c as the control parameter and a threshold near 0.1 (where γ−1 < 1%). A particle becomes a continuum when contact time approaches the sound-crossing time.
Many-to-many relations are drawn with one representative edge. “Change to a statistical description” can start at L0 or L2 and end at L3 or QSTAT; the graph shows two of those, and the node card lists all of them.