Template:Citation Style documentation/registration
Citations of online sources that require registration or a subscription are acceptable in Wikipedia as documented in Verifiability – Access to sources. As a courtesy to readers and other editors, editors can signal the access restrictions of the external links included in a citation.
Four access levels can be used:
free: the source is free to read for anyone (this applies in particular to articles in open access)
registration: a free registration is required to access the source-
limited: there are other constraints (such as a cap on daily views) to freely access this source 
subscription: the source is only accessible via a paid subscription ("paywall")
As there are often multiple external links with different access levels in the same citation, these values are attributed to a particular external link.
Access level of |url=
Links inserted with |url= are expected to be free to read by default. If not, editors can use one of
|url-access=subscription|url-access=registration|url-access=limited
to indicate the relevant access restriction.
Access level of identifiers
Links inserted by identifiers such as |doi= are not expected to offer a free full text by default. If they do, editors can use |doi-access=free (in the case of |doi=) to indicate the relevant access level. The following identifiers are supported:
|bibcode=with|bibcode-access=free|doi=with|doi-access=free|hdl=with|hdl-access=free|jstor=with|jstor-access=free|ol=with|ol-access=free|osti=with|osti-access=free
Some identifiers always link to free full texts. In this case, the access level is automatically indicated by the template. This is the case for |arxiv=, |biorxiv=, |citeseerx=, |pmc=, |rfc= and |ssrn=.
