Substring function

Hi,

I had created this function, Substring([contact_history_vw/@mktg_promo_cell_key], 13, 6), for the following formula: Substr(MKTG_PROMO_CELL_KEY, 13,6).

The expression is expressed in Adobe Campaign as, "sub string of mktg_promo_cell_key for ita_contact_history_vw starting at offst 13 of length 6":

Not having access to the actual data field, "MKTG_PROMO_CELL_KEY,"  assuming that the values are CHAR, what does, "starting at offset 13 of length 6" mean?

Is this correct Substring function for the query formula, "Substr(MKTG_PROMO_CELL_KEY, 13,6)"?

Thank you.