There is a UG in Virgo, CSS 130108:132530-082009 is given the range 14.31 (4.59). I asked about this before but can't remember the answer. Please can someone explain what this means.. is it 14.31-18.90? or something else? Thanks.
Take a look at the CSS light curve link (available through VSX):
Indeed, in the case of CVs usually the maximum magnitude is indeed the outburst magnitude when the CV was discovered and the amplitude is the difference between this magnitude and the quiescent magnitudes. The Catalina list from where these objects are imported may present the data in this way and that is why they end up in VSX like this. Patrick may comment on this.
But don't take this as a rule because when you see an amplitude in VSX it may mean different things (I always prefer ranges to avoid confussion).
An amplitude is usually given when a proper range couldn't be determined. The maximum value might be a mean magnitude of all measures and the amplitude would be the difference between the extreme magnitudes recorded. Catalogues from surveys like ASAS-3 also present data that way.
This means that, for instance, in the case of EA-type eclipsing binaries, the magnitudes in the maximum mag. field are actually close to the real maximum because these stars spend most of the time uneclipsed. In the case of red variables, which happen to show sinusoidal light curves, the maximum value will be around the middle of its true range.
Some new variables which are discovered without using standard filters may also have an amplitude reported. In these cases, a mean standard magnitude from a catalogue may be included as the maximum magnitude and then the unfiltered amplitude is added. This at least give a better indication of the star's magnitude.
You don't have to take those magnitudes at face value.
E.g. if the star in question was an eclipsing binary and the maximum magnitude was taken from 2 APASS observations and one of them happens to be during eclipse we may have: 14.02V and 15.02 V and the APASS mean magnitude will be 14.52V. The observer measured a 1.0 mag. amplitude so the table will say: 14.52 V (1.0) clear. But the maximum magnitude is not 14.52 but 14.02 (or so).
So giving ranges instead of amplitudes is the best approach, but sometimes exceptions are justified, specially when working without standard magnitudes and when we import lists from surveys where the data are presented that way.
You will usually have an external link from VSX to the data source that will let you check for yourself.
In the meantime I have added a range to "your" Virgo star.
American Association of Variable Star Observers
Thanks for that explanation, Sebastian. I have to do something when the skies are clouded over, so trawling charts for information is an option :-)