But I understand that with limited resources only part of the data in the original records could be indexed. My question is, what (kinds of) information is contained in the un-indexed portion of the records?It depends on what the particular register recorded. (This was sometimes nearly completely unrelated to what the preprinted form called for.) Some examples: Hebrew/Yiddish/religious name, occupation, date and place of wedding announcements, or cause of death.
The "comments" section on JG's indexes often captures probably three-fourths of the non-primary information, such as wedding witnesses and parents' birthplaces; just about the only sort of data I can think of that doesn't usually show up there is the midwife on birth records.
But you don't actually need to deal in hypotheticals and generalities for this question: given that most of JG's indexes were based on FamilySearch microfilms, and said microfilms have now basically all been digitized and nearly all have been made available online, it is theoretically possible to track down an online image for nearly every index entry.
As an illustrative example, take my husband's grandmother's birth:
Name: KRONBERGER, Elza Lili
Date of Birth: 12-Nov-1890
Mother: Janka KREISLER
Town Registered: Budapest
Jaras: Local Gov't.
Town Born: Pest
Comments: Father b.Papa,mother b.Papa
Source (Film/Item): LDS 642971, Vol. 20
Given this index entry, my first step to finding the corresponding image is to look up the film number in the FS catalog (https://www.familysearch.org/search/catalog). I paste 642971 into the Film/Fiche Number field and click Search. In this case, it's just one catalog entry: the Jewish congregation of Pest. I click on that and search the page (ctrl-F) for 642971 to see that this microfilm corresponds to digitized film 4466944, containing births 1890-1892. I click the camera icon by that line to browse the film, which contains nearly 600 images. They're in chronological order, so November 1890 should be toward the end of the first third somewhere; for lack of a better idea, I plug 140 (from the Record# field) into the image number, and notice that the page has a stamped 136 in the top corners. Hmm. Add four, and voila:
Film 004466944, image 144 of 597, top line (corresponding to the -01 of the Record#).
Details missing from the JG index: entry number, whatever is in Hebrew below the name, time of birth (!), legitimacy, father's occupation, street address, and midwife. Of these, the occupation and street address can be useful for identification, and the entry number is useful for constructing a "portable" (i.e. not hosting-site-dependent) citation of the register itself, but most of the rest of it is in the realm of "interesting trivia".
Note that it's usually not really possible to go from whatever's in the Record# field to an image number. For example, said grandmother's oldest brother has 293-01 in that field, but this corresponds to image 611 of 621 (on digital film 4227637). I find it most useful to go by the date, although you have to take each film individually -- sometimes, they're archive copies that have "booklets" of births, then marriages, then deaths for year N, then births, marriages, and deaths for year N+1, etc. And sometimes the films are of loose pages assembled in nearly-random order, with no guarantee that the recto goes with the verso that it has been paired with....