Maintaining costs mean you have a vehicle in the garage that's ready to run. Sustainment costs additionally cover all "other" lifecycle costs such as training crews, procuring e.g. simulators or other training and testing equipment, incremental upgrades to the vehicle as required, possible cost of intentional changes (example: personnel and equipment for mounting add-on armor), foreseen fuel and transport costs for intended operations etc.
I've never heard of that separation. Maintenance is an activity - but maintenance cost is only part of sustainment and we'd never mix the two, except
maybe in a SPO. Money for a fleet is split into two parts, acquisition (buys the kit, facilities and initial spares) and sustainment (buys everything else). Sustainment doesn't include personnel (for operating or logistics), just the fuel, ammo, spares and support contracts. Furthermore, depending on what upgrades you are talking about, they may be separate acq/sust budgets. As a simple example, M113AS4 was a separate project to the M113A3 purchase. So we took the M113 sustainment line, paid for bits (acq) to turn it into the best 1980s kit evah! and then added the sustainment funding to the M113 line.
To answer
@Bob53's question, the sustainment agreements between CASG and the Services are based on type, not platform. So, sticking with the IFV, there is a APC/IFV funding line now of $x. That's the sum of all the IIP sustainment lines for M113 until now. Lets say it's $s (for SUPER!) per year. The L400-3 sustainment line is an additional $e per year, after a growth period (of $a, $b, $c, $d as the fleet ramps up in size). So you have a budget that looks like:
s
s
s
s+a
s+b
s+c
s+d
s+e
s+e
s+e
s+e
In the Brigades you'd have (in theory)
s 100 AS4
s 100 AS4
s 100 AS4
s+a 80 AS4 + 10 IFV
s+b 60 AS4 + 30 IFV
s+c 40 AS4 + 50 IFV
s+d 20 AS4 + 70 IFV
s+e 100 IFV
s+e 100 IFV
s+e 100 IFV
s+e 100 IFV
And this is why it's important to get rid of old kit. Because CASG manages on capabilities and not platforms, every dollar going to an AS4 after the IFV starts delivery is one dollar less for the kit that can fight. It's when we have $s+e supporting 100 IFV and 20 AS4 that problems start arising. It was one of the main reasons we should have got rid of Kiowa earlier (it's a reconnaissance helicopter funding line) and ideas to keep AS4 around have real financial risk.
As for ASPI's article - *sigh*. Can anyone think why a IFV may cost more to sustain than a M113? I mean, it's not really heavier, bigger, has a more powerful engine, has air conditioning, a stabilised gun, more munitions and a digital fire control system - does it? Even ASLAV compared to Boxer isn't that applicable....
Furthermore, Hellier gets wrapped up around out turned v constant. His maths breaks down because of this, which complicates his already simplistic view. Urgh....
Yes, sustainment costs more. Yes, platforms are going to need more. But what other solution do we do? We can buy less, not maintain or realise this is the cost of doing business. I'm kinda proud the government took the latter.