version - "Version must be parseable by node-semver, which is bundled with npm as a dependency. (
npm install semver to use it yourself.) "
tag - The only tag with special significance is "latest". That version is installed by default when no other tag or version number is specified, and always points to the most recently updated version of a package.
Ill interpret version as a numbering system that accommodates range dependencies, whereas tag acts more like an alias. Tag could support a nomenclature like latest for most recent and stable for currently supported version for example.