General Order 24
General Order 24 is a Starfleet code that instructs the commander of a starship to use the ship's weapons to destroy a planetary civilization by orbital bombardment. Exact details are not clear, but the order apparently requires the destruction of all significant structures on the planet, be they cities or smaller installations such as military bases or industrial sites.
"This is the commander of the USS Enterprise. All cities and installations on Eminiar 7 have been located, identified, and fed into our fire control system. In 1 hour and 45 minutes, the entire inhabited surface of your planet will be destroyed. You have that long to surrender your hostages."
--TOS "A Taste of Armageddon"
It goes without saying that the need for Starfleet to include General Order 24 in its officer training program says a lot about the militaristic nature of Starfleet in the 23rd Century. While they describe themselves as peaceful explorers, they have already foreseen circumstances in which they might see the destruction of a planetary civilization as necessary and have pre-conditioned their starship commanders and crews to accept such an order, if given. In fact, an individual starship captain can issue such an order at his own discretion, without consulting Starfleet Command for approval.
In Versus Debates
General Order 24 is functionally equivalent to the Imperial order Base Delta Zero or the Imperium's Exterminatus order. Trekkies are prone to argue that this order demonstrates the superior firepower of Federation starships, since a single Constitution-class starship was expected to carry out this order against the reasonably advanced planet Eminiar VII.
Unfortunately for Trekkies, the statements in "A Taste of Armageddon" aren't specific about the degree of destruction required or the time spent in the attack, and Eminiar VII was actually poorly defended, since they engaged in war with their neighbors using only simulated weapons. A starship armed with dozens of missiles with nuclear yields and beam weapons capable of delivering terawatt-level firepower would certainly be capable of destroying a planetary civilization, but the extreme levels of destruction implied by the Imperial Base Delta Zero order are not required for General Order 24 (GO24 requires destroying the planet's civilization; BDZ requires eliminating all life and making the planet completely uninhabitable).
In "Mirror, Mirror", the ISS Enterprise orbited the Halkan homeworld, locking onto Halkan cities as they came into phaser range. This operation indicates that General Order 24 would be carried out by systematically destroying major cities and then continuing to less significant targets until the planet's civilization was effectively destroyed. Targets were coming into phaser range every few minutes, and if they had chosen an efficient orbital path for the operation, then each city would presumably be destroyed within a few minutes. It should be noted that the Halkans were a peaceful civilization with limited technology, and a more militarized target would presumably require more time to defeat local defenses.
In any case, without at least a time frame for completion of the assault (Scotty's ultimatum only states how long until the assault will begin), the mere existence of General Order 24 does not demonstrate that Federation ships have greater firepower than any other civilization capable of massive orbital bombardments.
Also, had the destruction been as complete as BDZ, identifying and targeting specific objects would have likely been meaningless.