SQLJ.install_jar, SQLJ.remove_jar, and SQLJ.replace_jar, are a set of procedures in the SQL schema that allow you to store jar files in the database.
Your jar file has a physical name (the name you gave it when you created it) and a Derby name (the Derby identifier you give it when you load it into a particular schema). Its Derby name is an SQL92Identifier; it can be delimited and must be unique within a schema. A single schema can store more than one jar file.