If you use the Nokia 7210 SDK you can access the additional nokia apis (background light, vibra alarm, etc). If you use those functions, they only work, if the device supports it.
Also, the sdk provides an emulator, which should behave like the real device. SUN left open a lot of specifications, how some things have to work, so this is implementation dependent. Also Sun supports more than what is mandatory according to the specification (i.e they support file access, nokia doesn't).
You can use Nokia Java phone emulators and Nokia Developer's Suite for Java two ways with Sun ONE Studio:
1. Integrate NDS with Sun ONE Studio when installing NDS. All the functionality of NDS is available directly from Tools menu.
2. Just install the emulators to J2ME WTK wtklib\emulators library. They can be selected from Sun ONE Studio Explorer Window, Runtime tab. Just right-click the emulator and set it as default.
The first option allows you to run the code in several emulators at the same time.
Remember to create a jar file (MIDlet Suite) before running the MIDlet in emulators as they can't run the MIDlet class directly from source code as WTK emulators do.