The Different "Versions" of Pixelmon
Pixelmon is a popular mod for Minecraft, based around an even more popular "monster catching" game. It is essentially just a single mod that is added to a forge modded Minecraft server to make Pixelmon. Even though it's just one mod, it's a BIG mod. It's actually bigger then some entire modpacks made out of hundreds of separate mods. The developers of Pixelmon have spent a lot of time and hard work, to pack as much content and features into this mod as they can.
To play Pixelmon, you also need to use a special launcher that will make a matching set of mods on your computer so you can join the server (you won't be able to play using the normal vanilla Minecraft launcher).
Why it's confusing
There are three different launchers you can run on your own computer that can be used to play Pixelmon.
There are now two major "versions" of Pixelmon server files that you can install on your server. One launcher has added extra mods (like Biome's O Plenty), and the other two launchers are just using the single pure Pixelmon" mod still.
If a Server has a list of specific mods installed, your client will not be able to join the server unless you have those same exact mods installed. If your client is missing any of the server mods, you get a mod mismatch error, and it will not let you join.
You can have additional mods on your client, that are not installed on the server. They won't prevent you from joining, but most often they won't work unless they're on both the server and the client. The exception to this our the client-side mods, that affect graphics, and add things like mini-maps (all of which takes place only in your client).
The ATLauncher Version
The version of Pixelmon that is being hosted in the ATLauncher platform is a special version.
It has some additional mods to add more features to the gameplay. It includes Biomes O'Plenty, a popular mod that adds unique terrain to the world generation, Journeymap, which creates a mini-map, and some other mods that add additional content and commands to the base Pixelmon mod.
If you use this client, you could connect to any Pixelmon server version. The extra mods won't work though unless you connect to a server running the matching ATLauncher server files though, that include these extra mods.
The Client to Use
Install the ATLauncher program, and then create an instance of the Pixelmon modpack. You will need to start up ATLauncher each time you want to join and play on your server.
The Version to Install on Your Server
This version of the Pixelmon modpack is located in the "ATLauncher" section of the Minecraft modpacks that are listed in the One-Click Installer on Nodecraft's control panel. It is labelled "Pixelmon Reforged (ATLauncher Pack)".
The Technic Launcher Version
There is a version of Pixelmon also hosted in the Technic launcher. This version contains only the Pixelmon mod itself, with nothing extra added.
Install the Technic Launcher, and set up an instance of Pixelmon. You will need to start the Technic launcher each time you want to join and play on your server.
The Client to Use
You can use the Technic Launcher to create an instance, or the standalone launcher released by the Pixelmon Modpack developers.
You will not be able to connect to a Pixelmon server made with the ATLauncher version. You will receive an error stating that mods are missing from the client. The Technic Launcher version does not contain the extra mods that are in the ATLauncher version.
The Version to Install on Your Server
This version of Pixelmon is in the "Miscellaneous" modpacks section of the One-Click Installer, on Nodecraft's control panel.
The Stand-Alone Pixelmon launcher
And last but not least, the makers of the Pixelmon mod actually have their own launcher. It will only use the Pixelmon mod itself, with no additional mods added. You'll start up this version of the launcher each time you want to play and join your server.
The Client to Use
You can use the Technic Launcher, or simply download and add the Pixelmon mod to a Minecraft server, using a 3rd party standalone launcher (like MultiMC).
You will not be able to connect to a Pixelmon server that was made using the ATLauncher version server files. You will receive a mod mismatch error, stating that mods are missing.
The Version to Install on Your Server
This version also uses the version contained in the "Miscellaneous" section of the One-Click Installer in Nodecraft's control panel.
Support
We here at Nodecraft will be happy to answer any questions you might have about the different "versions" of the Pixelmon modpack. Just contact our live chat support at https://nodecraft.com/support, and choose the "Live Chat" option. Our customer support is open 12 hours a day, 7 days a week, from 10:00 A.M. to 10:00 P.M. Central US Timezone.
Pixelmon Wiki/Discord
The developers of the Pixelmon mod also have a wiki/discord, and are always willing to help with any questions or problems you may have, if you can't find an answer to a specific question anywhere else.
https://pixelmonmod.com/wiki/index.php?title=Server_installation