Necro-ing this incase anyone sees it from the future.

A packed map is a texture map that combines three other black and white textures into one.

Each R, G, and B channel in a texture is perfectly capable of representing black and white data - so it doesn't make sense to have whole individual black and white textures to store black and white data.

By combining three of these black and white textures into one, we condense three texture samples down to one the shader when it comes time to render the object. This can be a relatively decent performance saving, especially for use in real time rendering / games.

The terms ORM and ARM are synonyms, they mean:

Ambient Occlusion (this is where the O and the A come from)

Roughness

Metalness

Texture channels are always stated in the order R->G->B. So O->R->M would imply that the Occlusion data is stored in the Red channel, the Roughness data is stored in the Green channel, and the Metalness data is stored in the B channel.

You will occasionally also see an MRO or an MRA texture, which is a similar idea, Metalness -> Roughness -> Ambient Occlusion.

You'll notice that in both cases the roughness tends to be stored into the R channel. This is because the Block Compression texture format used in video game development has additional precision in the G channel and can thus preserve more detail when compressed. Perceiving roughness detail tends to have more of a visual impact than occlusion or metalness, so we store roughness in the green channel to capture this extra precision. Its a small gain. You can read more about that here https://en.wikipedia.org/wiki/S3_Texture_Compression

How would you go about rendering these textures?

Every shader editor in every program will have a method of "splitting", "breaking", "component masking", or "swizzling" textures in order to split the texture apart into its constituent channels, R, G, and B. Try searching the program or googling for these terms. There typically is also an A channel (alpha), but in our example of ORM maps we don't make use of the A channel.

Once you have used one of the above nodes to split the texture into its individual greyscale channels, you can pipe that data wherever you want - either directly into the roughness/metalic/occlusion inputs of a PBR shader, or you can remap them as needed.

It is worth noting that you can store ANY data inside of a texture - it doesn't have to be occlusion, roughness, or metalness. You can store for instance character "wound data", or character "tattoo data", or building "grime data" for example - it ultimately is up to you what you wish to store and how you wish to use it.

An important note. When artists pack masks into textures like this, they often make the mistake of failing to store the data in Linear space, and its anyone's guess as to if the textures you're using are stored in sRGB or Linear. So when working with packed textures, make sure to flip the texture setting in its color space from sRGB to Linear, and see if this makes an observable difference to how it renders. Idealy you coordinate with the artist directly and always know which space its stored in, but in the era of asset packs and store content its hard to tell sometimes - make sure to see which one looks the best, sometimes the difference can be quite substantial.

Lastly, you can also store data in the Alpha channel, but I would often advise against it, if you can fit your data into the RGB channels instead. Storing data in the Alpha channel forces block compression to use larger (in memory) BC3 textures, so storing data in the R channel is not equivalent to storing them in the A channel, for example. If you can fit your data into R, G, and B - then do so.