Looking at the GEDCOM created from my RM file, it appears that the source that is being transferred through TreeShare to my Ancestry tree comes directly from the 0 @S1312@ SOUR, 1 TITL and the two continuation lines 2 CONC shown below. This matches the source portion of the long footnote in RM.
0 @S1312@ SOUR
1 ABBR Finland, Oulu, Kuusamo Church Records, births & baptisms, 1837-1855
1 TITL Kuusamo seurakunta [parish] (Kuusamo, Oulu, Finland), Syntyneiden luett
2 CONC elo [List of births], IC: 5, 1837-1855, (also includes marriages and de
2 CONC aths).
The citation appears to be coming from the PAGE tag for this source shown below, which is 548 characters excluding the 7 characters of 3_PAGE_. This was all on one line in the GEDCOM and was built from the individual fields in my citation separated by semi-colons.
2 SOUR @S1312@
3 PAGE birth / baptism entry for Johan Kallungi, 28 January 1846 / 20 February 1846, unnumbered pages arranged chronologically by baptism date; Kansallisarkisto [National Archives of Finland]; Digitaaliarkisto [Digital Archives]; browsable images; Digitaaliarkisto ||http://digi.narc.fi/digi/puu.ka; http://digi.narc.fi/digi/view.ka?kuid=5583057; downloaded; 24 September 2021; > Kuusamon seurakunta > Kuusamon seurakunnan arkisto > Syntyneiden ja kastettujen luettelot > Syntyneiden luettelo 1837-1855 (IC:5); image 77 of 215
What seems to be transferred to my Ancestry tree is shown below, which is truncated at 256 characters not counting the 7 characters of 3_PAGE_.
2 SOUR @S1312@
3 PAGE birth / baptism entry for Johan Kallungi, 28 January 1846 / 20 February 1846, unnumbered pages arranged chronologically by baptism date; Kansallisarkisto [National Archives of Finland]; Digitaaliarkisto [Digital Archives]; browsable images; http://digi.nar
It appears to me that the SOUR tag and the following TITL and CONC tags may be used to transfer the main source, and the PAGE tag is used for the citation by TreeShare. It would seem that Ancestry truncates the PAGE tag to 256 characters. To me it seems natural that the PAGE tag for the citation could be created by RM to follow the same logic as the SOUR, TITL and CONC tags. My thoughts are that if the PAGE tag size could be increased by Ancestry, then all of the additional information used to create long Evidence Explained source/citations could be added as part of the citation portion. Then everything could be transferred with SOUR and PAGE tags.
As a side note, the GEDCOM file has a FOOTNOTE tag which matches exactly with the long footnote template from my RM file. All of the field names are also found in the GEDCOM file.
1 NAME __Church Records (online images)
1 DESC Church records; church & series as lead element
1 CAT [EE, QC-7, p314]
1 FOOTNOTE <[Church_Author]>< ([Location])><, [RecordSeries]><, [RecordBookID]><, [
2 CONC RecordType]><, [ItemOfInterest]><; imaged in< [WebsiteCreator], > [W
2 CONC ebsiteTitle]><, [ItemType]><, ([Permalink_URL] :>< [AccessType]>< [
2 CONC AccessDate])><, path: [Path]><, [Image_ID]><; citing [Citing]>.