About Dlclist.xml

Although not strictly a mod, not in the "download/install" sense, this file is indeed modified and is one of the most important files for modders. When properly installed in the Mods folder, it replaces the function of the game version of dlclist.xml and tells the game to load your addons such as vehicles, peds, maps, and MLOs through OpenIV.asi - these are the mods contained in dlc.rpf packages.

This critical file will also include the vanilla game DLC (downloadable content) by default, which optionally you can remove if you choose to, with caution, as game/vanilla DLC can impact other mods.

There are actually 2 versions of dlclist.xml in the vanilla (unmodded) game. The lesser known one is found at the path root folder\common.rpf. They should not be confused. That one is unimportant for modders and contains just a few "platform" entries, for example:
<Item>platform:\dlcPacks\mpBeach\</Item>

Note: by root folder, we mean the game folder where GTA5 is installed, regardless of where purchased. The folder containing GTA5.exe.

The better known dlclist.xml, the one relevant for modders, was introduced as Rockstar released dlc for GTA Online. It is located at root folder\update\update.rpf\common\data and is only editable with OpenIV because the update.rpf which contains it is encrypted.

This is the version of dlclist.xml that is copied to the mods folder as part of update.rpf, either manually or by OpenIV's editor, resulting in a third version of dlclist.xml. It is located at root folder\mods\update\update.rpf\common\data. And, most importantly, this is the version you will edit to add entries for new mods.

Note: that some mods, particularly OIV installers, will add a 4th version in your mods folder. If this occurs, you will find this final version at root folder\Mods\common.rpf

When adding entries to dlclist.xml you need to exercise caution to ensure the xml syntax is correct, with proper open and close tags and proper spelling. In GTA5 SP (Single Player) the folder name (the one containing dlc.rpf) is always the one used in dlclist.xml. In the case of vehicles, the name to spawn the vehicle is generally the same as the folder name, but there are unfortunately exceptions.

DLC can be stored anywhere in your root folder, but by default, it will be in a folder called dlcpacks at root folder\Mods\update\x64\dlcpacks

A typical entry in dlclist.xml, which is an xml text file, will look like this:

 <Item>dlcpacks:/cobra/</Item>

With experience and attention to detail, you will avoid creating errors in dlclist.xml or at least know how to identify and fix issues.

An excellent tutorial on proper use and common issues can be found here Tutorial - dlclist.xml Troubleshooting/Syntax Rules. Strongly recommended reading.

Note: Contrary to what many modders put in their instructions, Windows folders are not case sensitive. This means you can call your Mods folder mods, or your Scripts folder scripts or other combinations. This is not necessarily the case for xml. As you will see if you read the tutorial linked above, you can have item or Item as valid xml but open and closing Item must have the same case, they must match. Similarly, if you spawn dlc (peds or vehicles) with your Trainer, you can use whatever case you want. Lara or lara, mustang or Mustang, either will spawn.