Each relationship is designed by the individuals in accordance to their needs and desires. Typically the couple will have many conversations discussing their needs and desires before potentially creating a written agreement / contract between them.
Although, not every couple has a written agreement/contract. It is a personal choice like every aspect of the relationship.
A written agreement/contract is not an absolute must have.
However, it is an asset to the relationship. It assist the couple to have a blueprint of their wishes as they agreed to originally.
It is a guide to help them through their personal journey.
As the couple see fit they can adjust the agreement/contract in accordance to their personal choice.
Typically the agreement/contract has a time frame in which they have agree to look back over the agreement/contract or it is simply discussed at some point that they have advanced in their journey and they agree or the Female decides to make adjustments to the agreement/contract.
Again this will be decided by the couple in their initial setting up of the relationship.