The current guidance on versioning is that the namespace URI should remain constant for all minor revs of a major version. That's fine, but I think the added feature of having that URI be a URL to the latest schema version is more confusing than convenient.

The biggest problem for VOTable is that the schema now has to allow multiple version attributes (e.g., 1.3 and 1.4) to support backward compatibility, while at the same time allowing 1.4-only features like TIMESYS. This makes the schema somewhat inaccurate as it allows version="1.3" and TIMESYS in the same document.

I would be happy to remove the convenience of having the namespace URI point to the latest schema. For VOTable writers that want to specify the schema (not required), they may as well point to the specific version. They know the version since they already must specify the version="1.x" attribute.

That discussion is outside the scope of VOTable itself, so if that remains unchanged, I propose that we recommend using the specific version schema URL instead of the namespace URI if referencing the schema.

decimal years representations in 3.5 : usage of besselian or julian years for timestamps apart from making timeorigin un-useful (and then forbidden) also implies some inference on the timescale used (see the TIME WCS paper for references and discussion). I suggest to add the the following sentence at the end of "timeorigin" explanation: "When using calendar epochs written in yr or Ba, note that conventionally Julian years are tied to the TDB timescale and Besselian years to ET (written here as TT) (Rots and Bunclark et al., 2015)." By the way, on a related topic I discover that in 3.4 COOSYS, we don't explicitly constrain the representation of time in equinox and epoch attributes. This however clear in the xml schema. I suggest adding after "... the epoch of the positions if necessary", the following sentence: " Both equinox and epoch MUST be expressed in the Julian or Besselian calendar".

Make COOSYS and TIMESYS more consistent:

• State that COOSYS and TIMESYS elements MUST be defined prior to their first reference. Currently COOSYS is only a SHOULD, and only for that case of a reference via "ref".
• State that references to COOSYS and TIMESYS must be referenced via a "ref" attribute. Currently references to COOSYS are allowed to be implicit.