PAC3 gives you the ability to personalize your player model's look by placing objects and effects on yourself. You can go from putting just a hat on your head to creating an entire new player model. PAC works on any entity and can also be used as a way to make custom weapons and npcs for your gamemode easily.
You can wear your outfit on any server with PAC3 and everyone should be able to see it on you as long as they have the content you used. If you're using content from Episode 2 (which very few people have mounted) they might see your models as errors.
You can also load custom models (.obj), textures (.jpg and .png) and sounds (.ogg and .wav) from URL.
By default, PAC3 is in basic mode. You can change this by going to options enable advanced mode.
The Wiki for PAC3 can be found here
This addon is also on the workshop
If you're completely new to PAC check out the Beginners FAQ
If you know the basics check out the Intermediate FAQ
You can join the official PAC3 Discord Guild here