A portable document appears the same on different operating systems. In the case of TrueType, documents can be portable between Windows and the Apple Macintosh computer; this could also be called platform portability. If a document appears the same on the Macintosh and with Windows, it can also look the same imported into different applications on either platform.
Since the same TrueType fonts work on the Macintosh, in Windows, and on all devices supported by both systems, the same characters and metrics could be exposed for all applications. Currently, however, fully portable documents are not possible. Windows and the Macintosh computer have slightly different character sets. Even though TrueType fonts contain the default Macintosh and PostScript character sets, Windows does not give applications access to the Macintosh characters. Likewise, a Macintosh application cannot gain access to the Windows characters present in TrueType fonts. Document portability is also a problem with international document exchange. Localized versions of TrueType fonts will still be in use for both the Apple System 7 and Windows version 3.1, leading to further character-set incompatibilities when documents that use these fonts are transmitted to a system that does not have them.