Hi Michael,
To create this field assignment first you have to enhance following structure: /SCMTMS/S_TCSCALE_ITEM_DATA.
Add a new component SCAVAL_DOC_TYPE (or decide a name of your choice) and put /SCMTMS/TOR_TYPE as component type.
Then, you assign this 'SCAVAL_DOC_TYPE' as a field assignment in scale customizing.
Thanks & Regards,
Tarun Kumar