This is a question for anyone consuming the pom-* parents of the SciJava software stack.
We want to start versioning these POM parents according to consistent rules. (Unfortunately, right now, the approach is vague and potentially inconsistent .) We came up with the following two possible schemes, and would like your feedback on which one you would prefer.
== 1) SemVer ==
Every POM parent has three digits, X.Y.Z.
The X digit is "major" and when incremented indicates a breaking change of some sort: either A) plugin config changes requiring downstream changes, or more commonly B) major SemVer dependency version bumps (e.g., managed scijava-common version updated from 2.35.0 to 3.0.0). This would exclude 0.x and beta components though (so e.g. imglib2-realtransform could go from 2.0.0-beta-27 to 2.0.0-beta-28 without bumping pom-imglib2's X digit).
The Y digit is "minor" and incremented when dependency versions change in a SemVer-compatible manner, or when plugin configuration is added or improved.
For critical bug-fixes where a given POM parent is somehow compromised or broken, the third digit Z can be bumped before going to the next Y. (See e.g. the recent pom-fiji 5.0.Z series of releases.)
== 2) Bread crumb version trail ==
The pom-scijava parent would have a single version digit. The pom-imagej (which is the next POM down) would have two: the first in lockstep with its pom-scijava parent, and the second being its dedicated version digit. POMs which extend pom-imagej (i.e.: pom-scifio, pom-imglib2 and pom-fiji) would have three digits: the first two in lockstep with pom-imagej and the third their own. And so on down the line—e.g., pom-trakem2 would need four digits: the first three matching the pom-fiji parent and the fourth its own.
== Pros and cons ==
[PRO] Semantic meaning. You can reason whether a given POM is somehow "breaking".
[PRO] Succinctness. Every parent POM has at most three digits at any one time.
[CON] Lack of provenance. Not obvious which parent POMs go together without leaning on a tool.
[PRO] Clear provenance. You can trivially derive the chain of parent POM versions.
[CON] Lack of semantics. Harder to tell which POM parent releases might break backwards compatibility.
[CON] Aesthetics. More than 3 digits in a version string may not be desirable.
Note that either way, we are in the process of creating a scijava-maven-plugin goal to dump all the component version properties associated with a given parent POM.
Since either scheme is consistent and useful, we want to institute whichever scheme will annoy everyone less. ;-) So which do people like better: SemVer or breadcrumbs?
 The 5.x POM versioning approach was an attempt to achieve _both_ options above, but Mark & I realized today that the two goals are rather mutually exclusive. That is, we cannot keep POM parent versions fully in lockstep while also maintaining a SemVer versioning scheme.