May be one of the following, in order from easiest to use to most control:
None: A database of conesearch catalogs is downloaded from STScI. The first catalog in the database to successfully return a result is used.
catalog name: A name in the database of conesearch catalogs at STScI is used. For a list of acceptable names, see vo_conesearch.list_catalogs().
url: The prefix of a url to a IVOA Cone Search Service. Must end in either ? or &.
A VOSCatalog instance: A specific catalog manually downloaded and selected from the database using the APIs in the STScI vo.vos_catalog module.
Any of the above 3 options combined in a list, in which case they are tried in order.