Technical Details
Like the 8-bit code commonly called variable-length quantity, UTF-9 uses a system of putting an octet in the low 8 bits of each nonet and using the high bit to indicate continuation. This means that ASCII and Latin 1 characters take one nonet each, the rest of the BMP characters take two nonets each and non-BMP code points take three. Code points that require multiple nonets are stored starting with the most significant non-zero nonet.
UTF-18 is a fixed length encoding using an 18 bit integer per code point. This allows representation of 4 planes, which are mapped to the 4 planes currently used by Unicode (planes 0-2 and 14). This means that the two private use planes (15 and 16) and the currently unused planes (3-13) are not supported. The UTF-18 specification doesn't say why they didn't allow surrogates to be used for these code points though when talking about UTF-16 earlier in the RFC it says "This transformation format requires complex surrogates to represent code points outside the BMP". After complaining about their complexity it would have looked a bit hypocritical to use surrogates in their new standard. It is unlikely that planes 3-13 will be assigned by Unicode any time in the foreseeable future. Thus, UTF-18, like UCS-2 and UCS-4, guarantees a fixed width for all characters (although not for all glyphs).
Read more about this topic: UTF-9 And UTF-18
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