What's the use of $_SYS_SHADOW_$?

Hi all,

I am using datatype shorttext in a SAP HANA columnar table. Getting the corresponding details from M_CS_ALL_COLUMNS shows, that for the column with data type 'shorttext' an additional, internal column $_SYS_SHADOW_<column_name>$ exists (which by the way occupies lots of memory).

What's the purpose of this 'shadow column'? Is it always generated autmatically for shortext data type? Are there other data types which also lead to  'shadow columns'?''

Thanks.

Cheers, Stefan