Do you miss having "pack modes" like in previous 1.7 modpacks? Well you're in luck!
PackMode is a very simple mod that gives you exactly that... pack modes!
CraftTweaker in 1.12+ works a bit different than it used to in earlier versions and as such, you CAN NOT change the modpack's PackMode modes "on the fly." To change the PackMode mode you must edit the packmode.cfg file to your desired mode, and then FULLY restart your client/server.
The way this works is by only loading certain CraftTweaker zs (script) files based on how they are "flagged" for modes (referred to as "preprocessors").
For example, if you put `#packmode expert` in a script file, it will ONLY be loaded if the PackMode config is set to "expert". You can also specify multiple valid pack modes this way: a script with `#packmode expert superduperexpert" will have that script only load if the PackMode mode is set to "expert" or "superduperexpert" modes, but will NOT be loaded if the PackMode mode is "normal" (or any other PackMode modes that might be configured). If you **fully** omit any kind of `#packmode ...` line in a script file, the default is that PackMode will not intervene and those scripts files will be loaded for ANY/ALL modpack PackMode modes. **If you do not use `#packmode ...` in any of your scripts, this mod will not do anything!**
The PackMode config allows you to configure as many different pack modes as you'd like, as well as which PackMode mode is default. Standard configs on PackMode's first install will default to include two PackMode pack modes ("expert" and "normal") as well as setting the pack's PackMode mode to "normal."