Reminder - in all my tutorials, green text can be skipped by modding vets, and red text can be skipped by beginners. These are side notes meant to clarify something I imagine certain readers will have questions on, and if they are not targeted at your skill level, you can skip them or ignore them if they are over your head or seem obvious.
The bulk of this guide was pulled from the Guided Series tutorial "Your First Addon" but changed to focus on the one-time steps - it also covers some of the steps that were skipped in that tutorial. This will likely be updated in the future as more advanced addon registration features are added.
In order for your plugin to interact with Sim Settlements 2, you need to create what we call a controller quest, and a few records to organize all your data.
Much of this part you only have to do one time for your entire addon, and then telling SS2 about additional content tends to be mostly drag-and-drop.
The Constant box ensures that you as the mod author always have control of this variable. If that is not checked, then if you update it, player saves that have already loaded with your mod will keep the original value, making the Global basically useless for this purpose.
The rule of thumb is, if you want the value to change during gameplay, Const should be UNCHECKED, if you want the value to always match what you the mod author set it to, Const should be CHECKED.
Under Miscellaneous > FormList, if you filter for AddonTemplate, you will find dozens of formlists you can duplicate to inject various types of content into SS2.
It's very important you always leave the first record there and as the first record. This is what I call the FormList ID keyword (FLID), and it allows SS2 to direct your content to the appropriate systems.
In general, the pattern for registering addon content is always the same - finding the appropriate formlist from the templates like you did in step 1, and dropping all of your new items into it. Then you add your formlists to the MyItems property like you did in step 10.
Once you have created a formlist and added it to that script property for a particular type of content, all you have to do is drag and drop new items of that type to the formlist!
Unlike the original tutorial for Addon Registration, in this we have set you up with Version Control. Anytime you want to update your mod for a new release, or even for testing on a save you have already used, you will need to incrememnt the value of the Global you created in step 1.
So for example, if you play with your addon to test some things, and save the game and then add more content, you will open your Global and change the value to 2 (then the next time you'll change it to 3, and so on). Then when you load that same save, SS2 will check your addon for new content since the Global has become larger.
If the global does not change, SS2 does not know to search your addon registration for new content. This is a change from SS1 and allows startup to go signficantly faster!
Generally though, when testing, it is a good idea to always start from a clean save before you release so you can ensure you are seeing the same thing players will. Then after you release your mod, you will want to test both with a clean save, and with an existing save that has already worked with an older version of your addon - that way you can be sure new and existing players of your mod will have a good experience.
So long as you load up a clean save, or increment your Global, SS2 will grab your content a few minutes after you load the game and allow you to use it.
If you have a large number of other addon packs installed, the registration time can start to get fairly long on a clean save. So I'd recommend you disable everyone else's addons for your early development testing, and then only re-enable them later when you're doing final testing to prepare for release.
A Note to SS1 Addon Makers
SS2 was built from the ground up to optimize things like load times and registration with addon content in mind, in fact SS2 itself uses the addon registration system too. There is no longer a separate system for the base mod and addon content like in Sim Settlements 1. This ensures that we always catch issues in the code before they impact you and your users negatively as they will come out in testing of SS2 itself!
After the initial registration period, each start-up for the player will be significantly faster thanks to the new design pattern, in fact, if no new addons have been installed by the player, and no prior addons have incremented their globals since the player last saved - the entire registration period will be skipped on subsequent loads - giving the player instant access to all content!