STL files only contain numbers, and they do not contain any unit information. This means that if you export your model to STL, the numbers or the models, not units, are exported and saved.
This characteristic of STL files can be problematic when you handle slicers. For example, your model is created in inches and your slicer uses millimeters by default. To fix this, you can scale your model in the slicer or standardize the unit of measurement in your entire workflow.
To learn more about exporting files, visit: Export files.