Fig. 3.1 - click on Unit NPC

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Fig. 3.2 - click MAU Tuning
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Fig. 3.3, 3.4, 3.5 - click and drag your MAU Key to this slot. (Shortcut - shift+left-click key)

Fig. 3.6 - this is where you click to repair your MAU. %HP and repair button is encircled.
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The following are default MAU parts.
Headpiece: Goliath, Catapult
Body: Goliath, Catapult
Leg: Goliath, Catapult
Weapon: Goliath, Catapult
Additional Weapon: Goliath, Catapult
Backpack: Goliath, Catapult

Fig. 3.??, 3.??? - Ammunition Tab. Click on the left slot til it turns into a brighter yellow. then available ammunition for that slot is available so click to equip that kind of ammo. click confirm to purchase, if you dont, your MAU is unarmed.

Goliath MAU has 3 types of Default Ammunition namely
Light Energy(11k+-) - The Light Energy equips your Goliath chassis with a fast ASPD but deals low damage.
Normal Energy(22k+-) - The normal energy equips your Goliath chassis with a med ASPD and med damage.
Hydrogen Energy(33k+-) - The hydrogen energy equips your Goliath chassis with a slow ASPD but deals the max damage.

The Catapult chassis ammunition is based on what type of weapon your current Catapult is upgraded with. a Newbie Catapult starts with a Vulcan, therefore you can only equip Vulcan Ammunition.

Each main weapon ammo, no matter what type, is a 1000 rounds. Meaning, 1 purchase of that ammo is good for 1000 attacks, misses included.

At Unit PT40, you can now upgrade your M.A.U. with a sub-weapon. a Goliath gets a Net Gun which can pin an enemy for 10seconds and prohibits it from attacking back. a Catapult gets a Multi-Missile Launcher. the current available missile decreases defense and evasion rate. each sub-weapon ammo is only good for 10 usages. costs around 33k+- depending on the Tax Rate. Now the catapult can store extra missile ammo in its backpack (backpack is available at PT15)