I agree that we should be consistent at least within the country rather than doing it piecemeal.

With the hierarchical network=* format as used on road routes, the hierarchical level is more or less the number of colons in the value, though less complicated countries would probably prefer the other format (e.g., fr:national). But this differs from the three-letter acronyms, which ostensibly indicate a zoom level range rather than any hierarchy per se.

For what it’s worth, this changeset moved the three-letter acronym to network:type=*, which is one of the ideas for preserving the ability of less sophisticated renderers to infer a reasonable zoom level.