Subfield $c is not repeatable so you don't need the code to repeat it in the xslt.
Subfield $a and $b are grouped together. They way it does it now is you have a CollectionArrangement with $c, all the $a and all the $b. When it goes back to MARC it is going to group them the same way, all the a together and all the b together.
Exmaple ocn 10227986:
`
Organized in five series:
<subfield code="c">Series I. Minutes and correspondence, 1936-1942,</subfield>
<subfield code="b">alphabetical and chronological arrangement;</subfield>
<subfield code="a">Series II. Correspondence re protection of the corporate name of the Presbyterian Church in the U.S.A., 1936-1940,</subfield>
<subfield code="b">chronological arrangement;</subfield>
<subfield code="a">Series III. Correspondence with other judicatories, 1936-1942,</subfield>
<subfield code="b">arranged alphabetically by correspondent within category (synods, presbyteries, local churches);</subfield>
<subfield code="a">Series IV. Correspondence, Auburn Theological Seminary,</subfield>
<subfield code="b">arranged chronologically;</subfield>
<subfield code="a">Series V. Miscellaneous.</subfield>
</datafield>`
m2bf version:
`
Series I. Minutes and correspondence, 1936-1942Organized in five seriesSeries II. Correspondence re protection of the corporate name of the Presbyterian Church in the U.S.A., 1936-1940Series III. Correspondence with other judicatories, 1936-1942Series IV. Correspondence, Auburn Theological SeminarySeries V. Miscellaneousalphabetical and chronological arrangementchronological arrangementarranged alphabetically by correspondent within category (synods, presbyteries, local churches)arranged chronologically
`
bf2m version:
` Series I. Minutes and correspondence, 1936-1942Series III. Correspondence with other judicatories, 1936-1942Organized in five seriesSeries IV. Correspondence, Auburn Theological SeminarySeries V. MiscellaneousSeries II. Correspondence re protection of the corporate name of the Presbyterian Church in the U.S.A., 1936-1940arranged alphabetically by correspondent within category (synods, presbyteries, local churches)alphabetical and chronological arrangementchronological arrangementarranged chronologically`
Finally, you could fix this specific field, but there are a bunch of 3xx and 5xx that have this pattern where the subfields repeat.
https://github.com/lcnetdev/marc2bibframe2/blob/cdff84cf049834bb6f9035a773b1b8a9d4935e41/xsl/ConvSpec-3XX.xsl#L257-L301
https://www.oclc.org/bibformats/en/3xx/351.html
Subfield $c is not repeatable so you don't need the code to repeat it in the xslt.
Subfield $a and $b are grouped together. They way it does it now is you have a CollectionArrangement with $c, all the $a and all the $b. When it goes back to MARC it is going to group them the same way, all the a together and all the b together.
Exmaple ocn 10227986: `
m2bf version: `