[h323plus] Q.931 IE length

This question is not strictly related to the software, but I was looking at a network dump of the call setup and noticed that the UUIE length is 2 octects. However Q.931-1998:4.5.1 always refers to a 1-octect length for all var-len IEs and I can not seem to find anywhere in Q.931 or H.225.0 where a 2-octect length is allowed. Where is this rule stated? -- Alessandro