To create a Parameter, you can use the small arrow next to Dimensions as highlighted in the following screenshot: 

When you create a Parameter, Tableau automatically opens the Edit Parameter window, as illustrated in the following screenshot:

This window is the only place where you can change or configure a Parameter. On the top, you can specify its Name and add a Comment (visible when you hover over it). You can also change the Parameter properties by defining the Data Type and the Current value (the value that the Parameter will have when you finish its creation). For some data types, you can also change the default Display format. 

The last option, Allowable values, is a core element of a Parameter. As you know, a Parameter is not related to the dataset and can thereby take any possible value. With the Allowable values option, you can, however, limit the possibilities of what the users can enter. Let's spend some time learning a bit more about the three Allowable values options:

• The first option, All, allows all possible values to be entered.
• The second option, List, limits the values that a user can choose from a list. For each item, you can specify the value and the alias (in the Display As column). You can also add all the values from a field or paste copied values from your clipboard (from an Excel file for example). Here's an example of the usage of List:

  

• The last option Range, is available for Date, Date and Time, Integer, and Float. With Range, you can set a minimum, a maximum, and a step size. The user will only be able to choose a value from that range. Here's an example of the usage of Range:

Click on OK, and your Parameter is ready! How to use it?  Continue reading!