BUG - Can't update (or) change project affectation resources.

A resource has a capacity
Most of the time Capacity = 1 FTE (1 full time equivalent), but it may be more (if the resource is not a person but a team) or less (if the person work only partial time).

A resource is affected to a project, at a certain rate
If resources are not shared between projects, so rate will probably always be 100%.
But if resources are shared, then rate could be less than 100%. If a resource is equally shared between two projects, then each project should enter a rate of 50%. This will lead to control that planning for each project will not overtake rate capacity, so that first project planning its activity will not take all the availability of the resource.
Project affectation capacity is controlled on a weekly basis. This means that planning for a project (including sub-projects) will not be more than (Resource Capacity) x (Resource affectation rate) x 5 for a given week.

A resource is assigned to an activity, at a certain rate
By default, assignment rate is 100%. But it may be less. This means that planning will keep some availability for other tasks.
Assignment capacity is controlled on a daily basis. This means that planning for an activity will not be more than (Resource Capacity) x (Resource assignment rate) for a given day.