Resources allow operations to take place at work centers when they are present, and stop them taking place when they are not.

To add Resources click on the Create Resource button on the left toolbar, and then click on the screen. The Resource icon will appear:

The number 10 above the Resource indicates the number of resources available.

Resources are assigned to Work Centers by opening the Work Center Properties tab on the ribbon and selecting the Resources button. Click on Add and select the Resource from the list.

Highlight your resource to see the properties tab in the ribbon. Alter the number of Resource available to 3, and run the model. You will see the number above the Resource changing between 0 and 3 - the lower the number, the more resources are in use. Reduce the number to 2, and re-run the model. The Resource will now be working as a constraint, and restricts the number of items able to be processed.