+2  A: 

This defines a ByteArray for the use with fast memory access which currently unfortunately can only be used by Adobe Alchemy or using Joa Ebert's TDSI helper project or HaXe. For regular ActionScript this has no use.

http://blog.joa-ebert.com/2008/12/01/alchemy-actionscript-asc/

http://ncannasse.fr/blog/adobe_alchemy

http://www.unitzeroone.com/blog/2009/05/22/another-scream-on-flash-alchemy-memory-and-compilers/

Quasimondo