segoeui.ttf

TrueType font Reference

A .ttf file contains a font in the TrueType format. The glyphs are scalable outlines, thus text stays sharp at all sizes. Apple developed the format, and Microsoft brought it to Windows. Operating systems load .ttf files to show text on screen and in print. Hint data in the font improves the quality at small sizes.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.