Mapping X12 fields to IDOC fields

Hi ,

<b>I'm creating mapping specs for 850 for a trading partner .</b>

Now I want to know how we do find out the corresponding Idoc field for an ansix12 segment .

Ex For BEG segment , BEG01 is a field :

<b>BEG01 353 Transaction Set Purpose Code</b>

I know we use transaction we60 in R/3 but not able to locate any idoc field with description "Transaction Set Purpose Code" .

<b>Is there some other way ? How is it generally found out ?</b>

Thanks

Sharad