Contract terms

What do the terms DISPOSE of any of our rights or OBLIGATIONS actually mean?

I received a contract from an agency.

One clause says:

This contract is personal to you and to us, and neither of us may without the written consent of the other assign or DISPOSE of any of our rights or OBLIGATIONS hereunder.

I also have another contract with the same agency which markets a package I have written and pay it commission.

What does that above term exactly mean?

Once the contracts start and if I want to stop using my Ltd Company and use an umbrella company, do I have to seek permission?

Does this term also imply that one could not set-off monies due to the other party from the separate contract as would this imply a disposal of our obligations?

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Mark Brown