X* measure numbers in MusicXML files

[Jacques Menu]

Hello folks,

In a file that contains Beethoven’s Ninth Symphony, there are are number of 
measures with numbers such as:

         <measure number="X1" implicit="yes" width="192.85"> 

all of them implicit.

The 3.1 DTD says:

<!--
        The measure-numbering element describes how measure
        numbers are displayed on this part. Values may be none,
        measure, or system. The number attribute from the measure
        element is used for printing. Measures with an implicit
        attribute set to "yes" never display a measure number,
        regardless of the measure-numbering setting.
—>

The same occurs in the middle of measure 2, so to say, in the much smaller file:

46c-Midmeasure-Clef.xml
Description: XML document

which musicxml2ly translates to:

46c-Midmeasure-Clef_musicxml2ly.ly
Description: Binary data

in which there is no true X1 measure, but merely a '\bar ""' in the middle of 
measure 2:

On this example, Sibelius 7.1.3 produces a dimmed barline with '[2]' on top of 
it:

MuseScore 3.2.3 produces a produces a dimmed barline:

and Finale 2014 produces nothing special, as musicxml2ly does:

Are such X* measure numbers always denoting a kind of subdivision of a measure, 
and what is the best way to handle them?

Thanks for your help!

JM