Setting up a CAD file for printing or publishing (to PDF or any other media type) is usually done by creating layout sheets that have a defined paper size and host view(s) to the model space together with other information (annotations, plan footer etc.).

The scale of the model views displayed in a layout sheet is specified as a property of the viewport so that you operate with precise scale all the time.

For doing quick prints of a specific area of your model, you won't need to set up a layout sheet though and can still output a specific area or view with precise scale.

In The Print - Model dialog you will find Print scale settings at the bottom left corner of the dialog (in Windows).
On the Mac platform you'll find those settings as "Print scale" option in the dropdown menu in the middle of the Print dialog

The Scale values refer to this: 1 millimeter (or inch) on paper = x units in the model (meaning drawing units).

For specifying meaningful scale factors you need to know the drawing units you've used in your model. In the Print Scale settings you can choose between inches or millimeters as output (paper) units.
If the model space units do not match one of these 2 units, you need to factor in the conversion between drawing units and that selected output unit.

Example: If the drawing in model space has cm as drawing units, then 1:50 print scale means this:
1 (mm on paper) : 50 (cm in the model)  = 1 : 500 scale.