FiveM MLO Glossary: The Files and Terms Every Server Owner Meets

Open any interior resource and you will find a folder full of files with short, cryptic extensions. Knowing what each piece does makes installs smoother and turns vague bugs into problems you can point at. This glossary explains the terms you will meet most often, in plain language.

What "MLO" means in practice

In the FiveM community, an MLO is a custom interior that the game treats as a real indoor space. It is built from rooms joined by portals and carries its own lighting settings, so players walk straight in from the street with no loading screen or teleport. That sets it apart from two other things you will hear about: built-in interiors that ship with the base game and are switched on by script, and "shells", which are small standalone interior models usually spawned for one player at a time.

File types inside an interior resource

.ymap: A placement file. It lists which objects go where in the world, with their position, rotation and draw distance. An interior resource normally includes a ymap that places the building at its spot on the map. When two resources place objects in the same area, overlapping ymaps are a common cause of doubled walls or floating props.
.ytyp: Archetype definitions, meaning the description of the models a resource uses. For an MLO, the ytyp also holds the interior definition itself: its rooms, the portals between them and any entity sets. If the ytyp is not loaded, the interior tends to show up empty or not at all.
.ydr and .ydd: A ydr is a single drawable, which is a 3D model such as a desk, a door frame or a wall section. A ydd is a dictionary that bundles several drawables into one file.
.ytd: A texture dictionary, holding the images the models are painted with. Oversized, uncompressed textures are the usual reason an interior is heavy to stream.
.ybn: A collision (bounds) file. It is what players and vehicles actually stand on and bump into. A missing or misaligned ybn explains most "I fell through the floor" and "I walked through a solid wall" reports.
fxmanifest.lua: The manifest every resource has. Map resources commonly declare this_is_a_map 'yes', and some add data_file 'DLC_ITYP_REQUEST' lines so their ytyp files load.

Interior terms

Room: A named volume inside the interior. The game uses rooms to decide which objects to draw while a player is inside, and each room can have its own lighting.
Portal: An opening such as a doorway, window or archway that links two rooms, or links a room to the outside world. Portals control what you can see from where. A badly placed portal gives you the classic bug where the street vanishes when you look out of a window.
Entity set: A named group of props inside an interior that a script can switch on or off. Creators use them for upgrade levels, alternative decor or optional rooms. Scripts toggle them with natives such as ActivateInteriorEntitySet and then refresh the interior.
Timecycle modifier: Lighting and colour settings applied to a room.
Occlusion: Skipping objects that are hidden from view so the game does not waste effort drawing them. Rooms and portals are a big part of how an interior achieves this.
LOD (level of detail): Simpler versions of a model that are shown from far away. A building exterior with good LODs looks fine from across the street without forcing everyone nearby to load the full-detail model. The LOD distance decides how close a player must be before the detailed version appears.

The stream folder

The stream folder holds the asset files that players download from your server: the ymaps, ytyps, models, textures and collisions above. Every interior you add makes that download bigger, so the total size of your stream folders affects how long new players wait the first time they join.

Using the glossary when something breaks

Floating or doubled props: look for two ymaps covering the same area.
Falling through a floor: check the ybn is present and the resource started without errors.
Empty shell with no furniture: the ytyp may not be loading, or an entity set is switched off.
Blurry or missing textures: large ytd files may be pushing past the texture budget.
The outside disappears through a window: usually a portal problem the creator needs to fix.

For a full setup walkthrough, the install guide walks through adding an interior to a server from start to finish. When you are ready to compare finished interiors, you can browse FiveM MLO interiors by type and check each listing against the terms above.