The answer is ..... yes and no :)

The Pro Pack can be used to make avatars. Remember that to be effective they should be low polygon based. A good rule of thumb is under 2000. Posette is a little over 16000, Vickie just under 29,000. You can make avatars with these figures but when you load them into an online world be sure to take a packed lunch to eat whilst you wait for it to load. Its also best to prepare some good comeback lines to respond to the other users when their keyboards have finally unfrozen and they compliment you on your choice of virtual persona. :)

You can animate your avatar in the Pro Pack and export it to the Viewpoint format that the world builder requires to create the actual avatar file. You will then need to write the java script to make your animations work.

The AvLab program will enable you to create an avatar from a set of body parts, not I grant you limitless but with more than sufficient combinations and adjustment to create something personal. You can if you feel the world is ready for it add your face to it realistically mapped to the 3D head. Select from a wide range of animations, click the publish button and you are done. If you set the details in the preferences it will upload the full file set directly to your web site.

So is the AvLab essential for participation in Atmosphere worlds? .... Nope, you can go in with the default Adobe avatar or one of the many that are available from other users if they want to share.

Are AvLab and the Pro Pack the only way of building avatars.... nope, you can create them in the Adobe builder. You will be limited to using primitives or importing Viewpoint files if you have them.

As with many questions like these, the answer will depend a lot on what you want to do with the program.