number sign(#) in sed scripts

Hi.

In a simple sed script i have number signs - #. But i can't find any description on their functionality. One thing that i find is that # in sed scripts can mean comment if it is placed in the beginning of the line.
I thought may be this undocumented(for me) behavior refers to regular expressions - but in regexp # doesn't mean anything special.

Script like that

VAR="1 2"
echo $VAR|sed 's# # & #'

and we get output

1 & 2

In this simple example i understand how it works. But i want full description to be confident when i change scripts. Any links?

thanks

Wow.
I found. It is just a delimiter insteed of usual backslash. You can use any delimiter of regexp's in substitute command.
So the command

sed 's# # & #'

is the same as

sed 's/ / & /'

I should read sed & awk book for some chapters more.
Topic closed.