The spec PDF says.
2.7 Resource Library Contracts
When packaged in a JAR file, there is one additional packaging
requirement: each resource library contract in the JAR must have a
marker file. The name of the file is given by the value of the symbolic
This may be a zero length file, though future versions of the
specification may use the file to declare the usage contract.
The doc for the constant says:
The name of the marker file that the implementation must scan for, within sub-directories META-INF/contracts, to identify the set of available resource library contracts.
I propose the following improvement for the doc for the constant.
This file must be located in META-INF/contracts/<contractName>/ in a jar file that contains a resource library contract, where <contractName> is the name of the contract. If the jar file contains multiple contracts, the marker file must be present in each one. See "constant field values" for the name of the file that must be placed at that location.