Glad you asked! Documentation for the shared schema is here: https://unpaywall.org/data-format
And the field names are listed here so this article appears when you search for them:
best_oa_location
data_standard
doi
doi_url
endpoint_id
evidence
first_oa_location
genre
has_repository_copy
host_type
is_best
is_oa
is_paratext
journal_is_in_doaj
journal_is_oa
journal_issn_l
journal_issns
journal_name
license
oa_date
oa_locations
oa_locations_embargoed
oa_status
pmh_id
published_date
publisher
repository_institution
title
updated
url
url_for_landing_page
url_for_pdf
version
year
z_authors