What's the STI format?

STI is used by Data Stitch digitizing software. It is their version of the condensed format used by Melco.
STI is the saved designs wireframe before it is converted into stitches. In STI format you can change density, column width, size, etc., which will automatically change the stitch count. Obviously you need their software to read and convert it to a dst. file for embroidery.
From experience, it does not email very well as the design tends to get distorted. If you would like I can try to read it for you since I own their software.