Difference between 2 tier - single assignment single contract and 2 tier - single assignment multipl

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Hello,

 

which the difference between this two employement model:

- 2tier - single assignment single contract

- 2tier - single assignment multiple contract

 

In both i can add assiggment and insert for the two assignment different information of the contract. So in both the person can have more then one assignment and more then one contract. So which is the difference?

 

Thank you,

giulia