This chart listed  three typical models for different management/configuration methods of the range extender:

How many kinds of passwords you might come through with the above models?

Management Password

• Local login password (for http://tplinkrepeat.net or Tether APP)

For RE200_V1(or other older models that didn’t support the Tether APP), there will be only a local login password. After accessing http://tplinkrepeat.net for the first time, it will ask to "Create a login password". (If it requires a username at the beginning please type in admin)
Note: if you forget the local login password, you might need to factory reset the range extender by holding the reset pinhole till all the LED lights turn on and then off.

For TL-WA850RE_V7, since it could be set up through the Tether APP, you will also come across a step to create a Local Account which is just the same as the local web login.

• Remote/Cloud login password

For RE200_V2, it supports TP-Link Cloud Management through the Tether APP.
After the basic configuration for the first time, if you also enabled the “TP-Link Cloud” service on the management page, it will require binding to a unique TP-Link email ID and password. Furthermore, after binding to the TP-Link Cloud, the local login password is no longer usable and it will only ask for the password of the TP-Link ID when you try to access the web management page.
Note: If you forget the Cloud login password, there is no need to reset the hardware and you can click "Forget Password" to reset the password only for the TP-Link email ID.

Wi-Fi Password

Host Network Password vs Extended Network Password

• Many Range Extenders don’t support customizing their own wireless passwords. So the Wi-Fi password for the range extender is the same as the Wi-Fi password of the Host router.

• For the models mentioned here(Support Customizing Password and Encryption Type on Range Extenders), their firmware versions are updated to support customizing individual wireless passwords for the Extended Network. 

Note: If you forgot this customized password, it Is suggested to connect a laptop via an Ethernet cable and access the web interface to locate the Extended Network Password.