The override file has a simple whitespace-delimited format. Comments are allowed (denoted with a #).
package priority section [maintainerinfo]
package is the name of the package. Entries in the override file for packages not found in the tree of binary packages are ignored.
priority and section place the package within the release tree; these ought not to be found in the control file. If the package is found in a subdirectory of binarydir, that will be checked against section.
maintainerinfo, if present, can be either the name of a maintainer for an unconditional override, or else oldmaintainer => newmaintainer to perform a substitution.
The override files used to make the official Packages lists may be found in the indices directory on any Debian mirror.