English to Russian Law/Patents Law: Contract(s)

This Agreement contains the entire Agreement between the Parties with respect to the subject matter hereof supersedes all previous agreements, and understandings, communications, discussions or negotiations between the Parties regarding the subject matter of this Agreement with respect hereof and may not be modified except by an instrument in writing signed by the duly authorised representatives of the Parties.

However, a party may be precluded by its conduct from asserting the invalidity of additions or modifications not made in writing to the extent that the other party has relied on such conduct.

Помогите разобраться со вторым предложением, а то я не могу понять суть....