If you need to prioritize work item in your simulation based on their type there is an easy way to do this.  First of all make sure your work items are differentiated by type through a label, with the highest value in that label being the type that has greatest priority.

For example, if patients are treated in an A&E department based on the severity of their condition we could differentiate patients based on their severity levels by a label.

Next step is to decide which activity you want to prioritize work into (for this example it would the treatment stage) and double click on the queue linked directly before that activity.

Click on the additional ribbon associated with that queue, and on the left hand side there is a check box which says prioritize.

Clicking this box will open a dialog box with every label created in your simulation.

Select which label you wish to prioritize your work items on.  This will now mean the activity directly linked after the queue will select items with the highest label value first.