STL files only contain numbers, and they do not contain any unit information. So, if you export your model to STL, the models’ numbers, not units, are exported and saved.
This characteristic of STL files can be problematic when you handle slicers, for instance, if your model is created in inches and your slicer uses centimeters by default. To fix this, you can scale your model in the slicer or standardize the unit of measurement in your entire workflow. In Shapr3D, you can change your model’s unit of measurement to your slicer’s during the export process: