[OU] Identify/Mask as - differences?

[rolf-str at privat.utfors.se]

I've tried a search in the knowledge base at Operas website, and found 
information that Opera 9 kan Identify as, and Mask as, O/IE/FF. This 
has to do with whether "opera" can be found or not in the id-string.

I did not, however, manage to find what differences there are in 
functionality. I seem to remember from somewhere that identify/mask as 
also changes what code can be executed, for instance what kinds of 
javascript.

What can Opera 9 do identified as IE that it does'nt as Opera?
                     "                "             FF           
"          "          ?

What can Opera 9 do masked as IE that it does'nt as Opera?
                     "                "           FF           
"          "          ?

thank you very much,
Rolf