Caveat that there are still some tagging exceptions where e.g. London's Docklands Light Railway and Kuala Lumpur's LRT are tagged light_rail despite being light metros. In those cases I'd suspect the idea is to provide a contrast with nearby heavier metros (the Underground and MRT respectively), and maybe a deference to the name as well.

And to clarify, the Palma Metro is entirely grade-separated, with high-floor vehicles to boot, hence should be a metro/subway.

I strongly disagree with azakh-world. The key distinction between light_rail and metro that I've seen most frequently and is in fact articulated in OpenRailwayMap Aktiventreffen 2014 nr. 2, is the degree of grade separation – light rail usually has significant amounts of level crossings, while metros are usually entirely grade-separated. There are some exceptions still to this, for example the Ottawa O-Train being light rail despite being entirely grade-separated, but that one was built with further extensions with grade crossings in mind and uses low-floor light rail vehicles, while the Palma Metro uses high-floor vehicles typical of metros. The use of overhead electrification does not make a system light rail – overhead electrification is very popular on new metros these days due to technical advantages. Smaller capacity makes this a "light metro" a la Vancouver or Copenhagen, but that's still metro. I've never heard the smaller gauge used as a criteria either – see the Bilbao Metro in the same country, also 1000 mm.

I presume they're judging based on usage=main: "a railway with higher traffic and higher speed than branch lines, often double tracked and/or electrified, may be high speed, usually longer than a branch line". I agree with davide that it's a travesty that it makes Sardinia look like a railway void at first glance. And semantically a branch railway needs a connecting main railway to branch off of. Surely the Cagliari – Golfo Aranci line at least deserves the main label: it's double-tracked from Cagliari to San Gavino, being electrified past that to Oristano on account of high enough traffic, and as the line's name indicates it officially continues even further for a total of 300 km.

*frequency, not voltage sorry

The iD in-browser editor doesn't show this prominently, but please note that there are prefixed variants of electrification tags for when the electrification (or the line itself) is still under construction. This matters for OpenRailwayMap rendering as currently implemented.

Whatever the perceived ambiguity of usage=industrial (which I don't see), surely usage=main is completely unsuitable for this line.

Electrification works east of Nogent-sur-Seine hasn't even started yet, therefore most of the changes are incorrect. https://www.sncf-reseau.com/fr/electrification-ligne4-nogent-troyes