I have a question regarding resource allocation percentages on project tasks.

Scenario:
I created a project using XPP and assigned resources to it. In the Resource Allocation view, I can see the workload of the assigned resources, including both tasks from the current project and tasks from other projects. By default, resources appear to be allocated at 100% on their assigned tasks.

Question:
Is it possible to change the allocation percentage of a resource for a specific project task (for example, allocate a resource at 50% instead of 100%)?

If yes, where can this be configured?

Thank you in advance, 
Simona