There is a bit of confusion about this, and pretty much always has been.
The confusion seems to have increased in more recent times, with some operators not able to circle under any circumstances, the published minimum in the chart being lower than the accepted industry standard minimum for cat C (at least in Wellington) (1500/8km vs 2000/8km) and the often subjective area of "how much turbulence and windshear exists, and is it likely to make the circuit unacceptable"?
Basically the AIP states that any time a crew cannot accept a circuit, approach control should be informed of that fact. (Don't have the AIP reference, sorry. 1: I'm at home, and 2: That document is counter-intuitive to navigate these days.)
Once approach has been informed, the likely result is that a 175kt speed restriction is placed on the approach, commencing at 10DME final. Reason: Tower is able to provide the protections based on tables issued for each a/c performance category. (These tables can be complicated.) Tower is unable to make a value judgement of this; the separations applied are delegated for use, and it's an approach separation. The distances used have been worked out using the simulator, and where indicated, tweaked over time and real life experience.
You should interpret the "may require an aircraft to enter the circuit.." as "That's quite likely to happen unless you've told us otherwise," and if you'd
prefer the MAPP (but don't actually
need it) let us know; it may be available.
It should be simple. If the weather was always clear cut, and everyone had a similar performance, and there weren't terrain issues affecting the ability to turn off the MAPP at WN, it would be. Unfortunately, reality all too frequently intrudes.
Where approach has previously advised a speed restriction and then cancels it, it simply means that Tower has no departing traffic to go ahead of the inbound. Where "no speed restriction" is advised, without previous discussion, you can fly at the barbers pole to the threshold, if you want. (The subsequent overshoot would be fun to watch, I'm sure, but you get the idea.)
As stated above, when the arriving a/c is believed to be able to enter the circuit, the required space between it and a preceding departure may be based on visual means, rather than radar (min. 3nm, which means the "gap" can be anything from 4nm to 12nm.)) which increases runway capacity significantly.
There was a recent change in the AIP concerning the ATIS. Up until recently, the ATIS included a phrase like "All aircraft category A and B advise approach on first contact if unable to enter the circuit in the event of an overshoot." Terrible.Wordy. You could die of hunger waiting for the ATIS to cycle. (Anyone dying of hunger here reading this?) Now that requirement is published in the AIP instead, and it is up to the crew to decide based on the reported wx, and up to us to try and report the wx accurately.
There can be several different reasons for speed restrictions on approach generally; the circling status is one of them, so in answer to your question re "no speed via libri", I don't know.
Do you fly a '738, perchance?
Sorry to waffle. I'm like that on the radio, too.
