Having had a quick Google, Mega Volt Amperes short circuit (MVAsc), in the context of ETAP, appears to be the total apparent power available under short circuit conditions. This is required for the purposes of it's simulation. The unit for this value is actually Volt Amps and in the example given the scale is Mega (1,000,000), so you're value is actually 6GVA (6 giga volt amps).
The values are higher the closer you get to the source of the power.
As an example, at the output terminals of one of the 500MW generating sets at Ratcliffe-on-Soar power station, the power available under short circuit (phase to neutral) conditions may be somewhere in the region of 166MVA (it's a three phase generator set, so it's roughly 500/3 I believe). The station as a whole has a capacity of something like 2000MW (designed, I think it's actually more like 2800MW these days if I recall correctly from my time at E.On Engineering), so at a rough guesstimate, that could be a phase to neutral VAsc of 666MVA.
By the time you get to a house, it could be 276kVA (that's a prospective short circuit current of 1.2kA - kind of typical in my area - at a nominal voltage of 230v).
This reduction is a result of many factors... the network includes protective devices designed to protect it against faults, losses in conversions and the impedance of the network itself.
For the record, this is way beyond my comfort zone and is a best guess based on a very very very limited understanding of what you're doing and the software you're using. The numbers above are just meant to illustrate the point, they are in no way meant to be accurate and are, if I'm honest, probably way off because I don't understand the finer points of three phase generation, specifically generating set capacity in watts and how that relates to it's ability to deliver power to a fault measured in VA.
My advice, seek help from someone closer to home who knows the exact context of this. I'm hoping it's an entirely theoretical endeavour set in an academic context (i.e. you're at college and this is an assignment, in which case you should get help from your tutors) because if not, you should already know this and fully understand it before you embark on designing anything that could potentially be expected to switch fault currents as things can go horribly wrong very quickly even with the relatively low fault currents possible at the average domestic supply head.
