What is the need for Project --> Demand Related List on the demand form.

[rad2]

On the demand form in PPM. what is the need for the related list Project --> Demand.

I am unable to link projects when i click on the Edit button

[Mark Manders]

OOB it's not on the form, so probably someone added it to have a quick way to go to the project that was created from the demand.
The 'edit' just shows all projects and I think you are simply not allowed to write to the demand field if it's filled, hence failing the actual edit, since the related list is showing all projects where the 'demand' field is the current demand.

[Mark Manders]

Let me update that: demand is read only on dictionary level, so you aren't allowed to write to it.