How to Use "Actions"

Haaaa,I Need Some Real Help
For Example,I Have 2 Files with Tags:
File 1:

 Composer -Joe
 Artist        -Roe

File 2:

 Composer-<u>Does Not Exist</u>
 Artist       - Jack

Now I want to bring the tags to COMMENT(Tag Field) such that COMPOSER*|*ARTIST
I Did that.The Result:
File 1:

 Composer -Joe
 Artist        -Roe
 Comment  -Joe*|*Roe

File 2:

 Composer-Does Not Exist
 Artist       - Jack
 Comment -*|*Jack

The First File Is Fine,My Problem is in the second file,since there is no composer field in it,it simple came as Comment -|Jack, Since it doesn't have the composer field There Is An Unnecessary "|" In The Tag Field.How Do I Stop This,That Is,Create "|" Only In Files That Have The Composer Field,The Files That Don't have the composer field Should be simple have the artist tag alone.

You could filter you files first before you apply your action
NOT %composer% IS "" AND NOT %artist% IS ""

Or you could add a "Replace with regular expression" action that removes the superfluous characters at the beginning of the comment field
tag: comment
search: ^*|*
(leave replace empty)

Just one more note: using these special characters asterisk and bar makes it rather difficult to process the comment field afterwards as they have special meanings in mp3tag's command language
Also, if you ever intend to incorporate the comment tag in your filename, the special characters will be ommitted as they are not allowed as characters for filenames. It might be a good idea to think of other separators between the two pieces of data.

You should take into account what ohrenkino said :
"It might be a good idea to think of other separators between the two pieces of data."

Action: Format value