Default [Q] Full phone backup in twrp...

Hello aigan, I have taken a backup of my phone, but I didn't have enough space to take a backup of my data which was on whopping 3,2 gb (data only!) The total backupsize with data was on 6(!) gb! And I didn't have enough space to take a backup of that...So, what do I lose if I backup a backup with just system, boot, and recovery, and not data? Do I loose all my pictures, apps etc?

And, how to I delete backups from twrp?

Might be some noob questions,but I want to ensure what I am doing
"LG D802 with different custom roms!"
Changing roms too fast these days.....
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