Question: why a master "should" have axis value?

i have a question about master axis value requirement necessity. a typeface might have several styles and user wants to collect all of them in a single Glyphs source to sync their design and/or metrics… so sometimes whole project dividing into some sub-styles with their own axis(like Roman and Italic…) or single styles(like super heavy or texture…) without any axis. i mean master axis value seems should be an optional parameter to include or exclude it from being a part of a variable font in export and to counts/counts not in master compatibility check… so would it be possible at all? and why not? please clarify. thanks.

This is mostly because many internal parts of Glyphs (any many plugins) identify a master by its axis values.

For example, to a human it might be clear that a Filled and an Outlined master have the same width, weight, etc. and differ only on the outline style. To make this also clear to the software, you give both masters the same axis values for weight, width, etc. (whatever axes your font project has) and give the two masters different axis values for, say, the Style axis. 1 for Filled and 2 for Outlined, for example.

Now Glyphs understands how these masters differ and how they are similar. This machine-readable axis value information enables many of the features dealing with masters and is not only used in interpolation/variation.

i think this situation forces all masters to be in one island, my point is why Glyphs doesn’t allow to create more than one island? for example two Filled masters on a weight axis and one Outlined master without any axis? (without using Disable Master or in separated Glyphs source)

Because you would then need to define which masters belong to which islands. And that is done using separate Glyphs files, one per island.

i still think it’s simply solve with adding a N/A value to master axis value(i still discuss from concept view not technical) to expand the possibilities. maybe i have not a correct view about that.

You can do have more than one island. You need to give each point on your map a distinct coordinate. Just like real islands, that can’t be at the same place.

could you please elaborate with an example?

Let me explain what i meant from word ‘island’:
all masters in Glyphs app should have a valid value for any defined axis, this situation cause all masters linking together. this link creates one ‘island’
If we allowed to break the link between these masters(for example by deleting the axis value to remove one master from the axis) then we could create another ‘island’.

But how do you want to differentiate between masters, in that case? As Georg pointed out, each master needs to have unique coordinates in order to be able to pinpoint it in the design space and differentiate it from the other masters.

why we need all masters to participate in designspace? we may have some stylistic masters and do not want to appear in compatibility check(but still wants to collect them all in one source to sync their design, unicode support, metrics,etc). or need to subset by choosing axes in multiple variable font settings(create and customise more than one designspace)…

I still don’t understand the problem. But is it possible that it has to do with the compatibility indicator (the red bar in edit view)?

there is no problem, all good. this was just a question to understand the model. i just need to discuss about that. thank you @GeorgSeifert