You're better off using Surrogate Key (B) as opposite to Natural Key (A). The Surrogate Key is usually integer field with sequential autogenerated values. BTW, VFP8 natively supports such fieds. See http://fox.wikis.com/wc.dll?Wiki~SurrogateKey for more details.

>Please give me an advice
>
>
> Which kind of field is good to use as ID in the father table for creating the relation to the child talble ?
>
> A)The number of client, which is a field that can be edited
> by the user
>
> B)A hidden field that the user can't access