When you watch the digitized video of your film, every individual video frame will be created from a single, individual film frame. That means that you can take your digital file, slow it down frame-by-frame, and create a single still image from your film.
If you transfer your film with another company, it is highly unlikely that you will be able to do this. That's because film and video playback at different speeds, and most film transfers alter the frame rate of the film (which was usually shot at 16, 18, or 24 frames per second) and convert it to the standard video speed of 30 frames per second. To make this work, they add what's called a 2:3 pulldown, wherein two additional video frames will be created for every three frames of film. The artificial frames will be created by merging two frames together in a sequence.
By eliminating pulldown, not only will you be able to create a still from individual frames, but the film will also playback cleaner and clearer than it would had the merged frames been added.