1: Why is there a used memory?
10 mb is for logging. Device always logs 10 files each 1 mb of size.
With preconfigured logging filters.
last 3 mb - can be from last downloaded file to device. It can be either device configuration file or last FW update file.
So in your case i believe on SD card -> there are: 10x log files 1MB each + FW update file ~4mb.
2: Will it delete automatically? Or must we plug the devices to PC every few weeks and click on "Format" in the Teltonika Configurator ?
You dont have to delete that used memory. I very much doubt you would encounter a situation where you use up 100mb for data records.
1 record size varies - can be up to 1 KB maximum. But if less IO elements are enabled - then record size is smaller.
So lets imagine you have 100B of record size.
100 mb == 102400 KB == 104857600 B
converting to records count == 104857600 / 100 == 1048576
lets assume your device saves 1 record every 1 second. 1048576 seconds == 17476 minutes == 291 hours == 12 days.
Does your device ever go into such operational state - that it saves a record every 1 second for straight 12 days and during all that time does not have GPRS / GSM -> to send records to server?
I very much doubt so.
Also your device is very likely - does not have a configuration where it saves 1 record per 1 second. So if it saves less records -> it will last WAY more than 12 days.
Just some calculations.