The Recent Apps Key can be used for taking screenshots, splitting the screen, and displaying the menu.
Setting up the Recent Apps Key on Asus Zenfone 4 Max ZC520KL according to the following instructions:
Step 1: Go to settings and select Asus custom settings.

Access settings and choose Asus custom settings.
Step 2: Select the Recent Apps Key.

Select the Recent Apps Key.
Step 3: Choose the desired feature to use.

Select the feature to use.
Congratulations! You have completed the setup process. Wishing you all success.