Oracle ships the JDK with "JavaDB" which seems a renamed version of Apache Derby.

Is Java DB up-to-date and synced constantly with every update discharge of the JDK ("_uXX") or just for major releases like Java 7?

The Getting Began with Java DB guide addresses this within the note around the relationship between your two items:

Relationship between Java DB and Apache Derby

Java DB is really a relational database management system that's according to the Java programming language and SQL. Java DB may be the Oracle release from the Apache Derby project, the Apache Software Foundation's (ASF) free relational database project.

The Java DB product includes Derby with no modification whatsoever towards the underlying source code.

Because Java DB and Derby have a similar functionality, the Java DB documentation refers back to the core functionality as Derby.

Java DB Version 10.6 is dependant on the Version 10.6 discharge of Derby. References to "Derby" within the Java DB documentation make reference to the Version 10.6 discharge of Apache Derby.

In line with the sentences being stressed in bold, I'd conclude that every version seems just like the actual Java DB version, using the version amounts also being exactly the same (this really is going through the versions indexed by the Java DB downloads page).

This could not address the question on when Oracle/Sun constitutes a era of Java DB. I'd assume that's left towards the specific team that handles the Java DB releases.