The type of relationship specified is the only type possible with the data structure of the tables - a one-one rlationship can only be set-up between primary keys which match completely i.e. the number of PK fields have to match, if composite PKs are used.
If you're linking to a non-PK field, or only part of a PK then a one-many relationship will be possible. If you want a non-PK field to be unique, just set no duplicates, and then setup a one-many with the PK of the other table - which will act like a one-one.
Something I learned the hard way!?
