The PRI ALT is just a language concept. As far as I know never implemented.
This appears to be exactly backward. The PRI ALT is the only thing ever implemented. The priority depends on the order of disabling. See the Inmos Compiler Writers Guide.
The consequence is that it is really an ALT. Which means , don't assume anything about the order in which the ALT triggers. If not, you are likely to introduce side-effects in the code. Therefore the opposite is better. Always implement in order of priority but still assume that the "select" is priority independent. It keeps the behaviour consistent even if the timings (in a network) vary. Because in practice, one should not assume anything about the order of things on other nodes. Logical (functional) behaviour and timing behavior should be independent.