Power and volume down menu... What kind of trouble am I getting into? Model ZTE N9500 
Carrier:　Sprint

Messing around with various button combinations I found this one and up popped a very interesting menu my question is. What are the respective purposes / consequences of each choice .

1 ) reboot( self explanitory)


apply update from external storage.


3 wipe data/factory reset.

4 wipe cache partition

5 apply update from cache

6 md5 check

7 wipe carrier/data/factory reset.

Thanks in advance,
Chris
What you have found is the recovery mode (or recovery in short) which is useful for manipulating the OS of your phone.


Options 2 and 5 take a .zip file of a specific format and update the OS of your phone with it. This is called "flashing". Stock recoveries (most likely you have a stock recovery if you don't know what this is) usually verify if the .zip was signed by the vendor of your phone - so you can't flash homemade .zips using this and so this option is pretty useless for you.
Options 3, 4 and 7 erase various pieces of storage in your phone. 3 erases all of your data but preserves the OS, which is useful if you want to sell your phone or it has become unusable and unfixable from inside Android. It essentially resets your phone to the state it came from the factory in. 4 does no harm, it erases temporary files which help speed up Android. These will be regenerated at the next boot, and the phone will be a little slower for a while. It can help in some cases when apps crash. I do not know what 7 does, but the "wipe carrier" part seems dangerous enough for me to not try it ever. Best case, it does a factory reset, like 3. Worst case, it makes your phone unable to connect to the carrier and make or receive phone calls.
6 seems harmless enough, it seems to do an integrity check of some sort, maybe on update packages, maybe on the phone. You may as well try it, I don't think it does anything harmful or interesting whatsoever.


If you're interested, you can search for and install a custom recovery which will replace this menu with a much better one. This will allow you to install custom OSes to your phone, make and restore backups and much more.