Ok, so as a player who previously had some mod experience and touched on Forge a ton in previous years, there’s a very rigid line between the two, and what they can do is more or less opposite spectrums.
In the past, modding was more or less changing global game values to cheat in Matchmaking. Halo 2 was extremely plagued with bridgers and their modder friends to instantly (or nearly) win every match they played. It involved changing parameters like damage, weapon projectile speeds, headshot damage, the projectile itself, rate of fire, clip size…you get the drill. The cheating done with this also removed spawn points and adding existing assets to areas of maps that weren’t supposed to be there. This only worked when the modder got host box that everyone else connected to (hence the bridger needed).
As Forge released with Halo 3, then Reach, 4, and now 5, the lines have thinned, with scripting in Halo 5 paving the way as the only resourceful way to replicate modding from the old days. Scripting can really alter maps in ways that aren’t typical in everyday settings… Which leads me to my next point.
Modding is derived from “modification.” Any change is technically a mod. However, my firm belief in using the term is that the modification is the result of changing the game via an outside source or influence. I say influence because, in the past especially, exploits and glitches could cause a game to write values to ROM (read-only memory), which isn’t allowed and can result in crashes, the game not working/booting, etc. In the physical copy age, this could destroy a game beyond unplayability (look up Donkey Kong Country 2 Castle Crush glitch to see what I mean). Scripting allows mod-like attributes to happen to an extent (such as giving a Spartan a set of traits for doing something that gives enhanced shields, damage, and speed). With Forge getting more advanced, the features it has pulls away from modding. Modding itself has evolved over the years, and the support it gets on Steam and other communities is basically to the point of custom add-ons and custom DLC being created. The cloud and current gen games aren’t able to be modded and with everything starting to run from dedicated servers, matchmaking has essentially wiped modding in the form of cheaters from current generation titles.
So if we look at Halo 5, something modding can do that Forge can’t is the best way to look at it. Forge can’t alter firing rates of weapons. It can’t create dev-styled looking maps. It can’t create engine-ran, fan-created gametypes. Scripting can kind of do gametypes but it’s not as polished or stable as an actual gametype can be, with very limited options as well. An example of this, I ported over and converted Assault to work on Halo 4. It’s not a native gametype so what was done was a gametype mod. 343 released Mini-Slayer as an Action Sack gametype in Halo 4, and that was originally a mod the community made. Theods came from outside tools that were created by reverse-engineering and guesswork to understand what the data was doing. As more was understood, better tools come out to let the community mess with things themselves.
This is where I want to get Halo. Matchmaking and Social are already split. The ability for the community to mess with anything and everything locally should be fine, as long as it doesn’t affect other people playing. Visual aesthetic changes are generally the most socially accepted (HUD/UI alterations, upgraded graohics for 4K [if it doesn’t have it]).
It’s a mess but the rule of thumb is, if the devs didn’t release it and the game doesn’t allow you to do it under normal conditions, then it’s a good assumption to say it’s a mod.