Hey @MANND, TheBossa here!
So I created this some time ago while I was working on a game jam. Since then I have discovered more about it tho I’m not an expert on this topic. However, I’ll share with you what I know.
If you can it’s best to consult someone who is experienced in this area. Also if you find that person please let me know 
I’m assuming you’re asking this question cuz you had some issues animating. Normally you can create the Humanoid Avatar automatically by going to the Base Mesh and selecting the Humanoid option inside the dropdown. It will create a Humanoid Avatar inside the .fbx/base mesh file.
But Generic animations won’t work with these avatars because Humanoid Rigs use muscles which are controlled by Constraints. Here is a link made by Unity for more information: How to Work with Anims
So if your animations are Generic it means they are using bones therefore in your base mesh the rig option should be set to Generic. So either you create an empty animation or a pre-made animation, as long as the animation is Generic you should use Generic rig. For me, bones are easier to animate. That’s why I choose Generic animations (Generics has avatar too).
But you have Humanoid Animations and you want to edit or completely re-animate them. Then you simply need to use Humanoid Animations and Constraints like Transform Constraint or Two Bone IK. I normally find animating in Humanoid too chaotic as you can either use constraints on all bones or only on the important joints. I discovered that only use transform constraints in important joints to animate them or edit the pre-made animation. Here is an example:
ABOVE IMAGE SHOWS* Left 2 are using the same animation but you can see them doing different movements (Transform Constraint on Humanoid Rig). Middle Gray Zombie is using Two Bone IK for arm animation while doing an Idle animation. Anime Girl on the right is an empty animation only made with Transform Constraints on a Humanoid Rig.
You may ask me what about converting the animations or can’t I use both of them without any changes and edit the animations? Well in that case I only could find two paid store assets (which I plan to buy and try out) which are UMotion and Animation Converter. Here are the links:
Umotion Discussions
Animation Converter
On top of that I’m gonna give you some workarounds you might find useful 
Workaround 1: Upload the character to Mixamo, download the T-Pose animation with the skin, use that avatar on your character, and animate.
Workaround 2: Switch between Generic/Humanoid rigs. Record animation while in a Generic rig then use that animation in a Humanoid rig. Both support the avatar mask and IK.
Workaround 3: Create your animations in 3rd Party Software such as Blender, Maya or others.
Workaround 4: Use Generic only rig if you are comfortable with animating with bones or your animations are Generic.
Workaround 5: Use Transform Constraints in your Rig (Animation Rigging>Setup Rig) to edit your Humanoid Animations (bones will not work because it’s muscle-based).
Hope this covers everything!
Good luck and always feel free to reach me for questions!