Most of the time, when people say "multimission ship" they're talking about AAW or ASW or ASuW, the primary threat the ship is intended to face.
Not what the ship is supposed to be doing at a higher level, or how much help it's supposed to have while doing that job. Not "this ship is intended to be off by itself
so it needs greater fuel, stores and spares capacity than a ship that is intended to be operating as part of a fleet
with a supply ship right there."
Also, the modern ASW threat has enough missiles that an "ASW escort" needs to have some pretty competent AAW capabilities. Hence SPY6 and Aegis on the Connies.
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So, a Frigate. Intended for operations mostly off by itself, commerce protection, anti-piracy, convoy escorting, etc.
- Needs a long unrefueled range.
- Needs enough food stores to cover at least 90 days underway between pit stops, more is better.
- Needs a machine shop and enough spare parts to stay fully functional at least 90 days between pit stops.
- Needs a sick bay with xray capability, and possibly a surgeon onboard.
- Convoy protection means a helicopter is a good idea (I mean, a helicopter should be mandatory for all ships these days), and since the ship is intended to be off by itself we need two plus a hangar for them, as well as all the spare parts that they need for 90+ days away from their usual support.
- Anti Piracy means a couple of RHIBs and space for either a MARDET or a Coast Guard LEDET, or both.
Notice I haven't mentioned weapons or sensors yet, except for the helicopters.
In comparison, a Destroyer. Intended for operations as part of a fleet, and supporting other, larger ships.
- Will be refueled every week or so from the fleet oilers
- Has a supply ship steaming with the fleet, so doesn't need 90+ days of food
- Has a supply ship close by, so doesn't need all the spare parts, just those commonly used week-to-week
- Has a bigger ship close by, so doesn't need a surgeon onboard, or even the xray machine(s).
- Still needs a helicopter for SAR etc, but may not need 2 or even have a full hangar for them. Doesn't need all the space for helicopter spare parts or maintenance, either, since it will be operating with a carrier that has all that stuff onboard.
- No need for a MARDET or LEDET, but will need a couple of small boats for general use.