1 - Process to limit, through at least one processing and memorizing unit, the possibility to transform T X -type data into T Y -type data and the possibility to transform T Y -type data into T X -type data, the transformation of the T X -type data into T Y -type data being carried out using an A-type transformation function (F A ), while the transformation of the T Y -type data into T X -type data is carried out using a B-type transformation function (F B ), inverse of the A-type transformation function (F A ), the data being in particular designed to constitute for instance, pre-payment tokens, and being implemented on at least one data processing system, said process comprising: using an A-type data processing system (STD A ) and a B-type data processing system (STD B ), setting up at least once, at least one link (L) between the A-type data processing system (STD A ) and the B-type data processing system (STD B ), so as to provide the transfer of at least T Y -type data from the A-type data processing system (STD A ) to the B-type data processing system (STD B ) and/or to provide the transfer of at least T X -type data from the B-type data processing system (STD B ) to the A-type data processing system (STD A ), during an A-type customization phase, creating at least one A-type processing and memorizing unit (UTM A ) including at least the A-type transformation function (F A ), during an A-type transformation phase: for a user possessing at least one A-type processing and memorizing unit (UTM A ), enabling: to transfer at least one piece of T X -type data from the A-type data processing system (STD A ) to the A-type processing and memorizing unit (UTM A ), to transform in the A-type processing and memorizing unit (UTM A ), each piece of T X -type data into a piece of T Y -type data, using the A-type transformation function (F A ), to transfer each piece of T Y -type data from the A-type processing and memorizing unit (UTM A ) to the A-type data processing system (STD A ), for a user not possessing any A-type processing and memorizing unit (UTM A ), not being able to transform a piece of T X -type data into a piece of T Y -type data, using the A-type transformation function (F A ), during a B-type customization phase, creating at least one B-type processing and memorizing unit (UTM B ) including the B-type transformation function (F B ) and not including the A-type transformation function (F A ), and during a B-type transformation phase: for a user possessing a B-type processing and memorizing unit (UTM B ), and not possessing an A-type processing and memorizing unit (UTM A ), enabling: to transfer at least one piece of T Y -type data from the B-type data processing system (STD B ), to the B-type processing and memorizing unit (UTM B ), to transform in the B-type processing and memorizing unit (UTM B ), each piece of T Y -type data into a piece of T X -type data, using the B-type transformation function (F B ), to transfer each piece of T X -type data from the B-type processing and memorizing unit (UTM B ) to the B-type data processing system (STD B ), not being able to transform a piece of T X -type data into a piece of T Y -type data using the A-type transformation function (F A ).