BMS for Li-Ion Batteries

Some BMS for Li-Ion batteries control the SOC battery 20%-80%. I bought a BMS from Aliexpress model JBD-SP15S001 use for my Li-ion battery. I heard BMS will control the SOC for the battery. When I fully charge the battery, the LCD read 100%. Does it mean the battery only 80% full (because BMS prevent the battery charge to 100% to prolong the life of battery). Same thing when I discharge the battery , LCD show 0%. Does it mean the battery has no more juice ? Does it mean 0% or still has 20% to prolong the life of battery ?

SOC reading is generally of little value.
Cell voltage tells all on most chemistries. Exception being LiFePo4 which holds the same voltage through a large range.
If you post details and pictures, you will get lots of help.
It seems you may have some misinformation already.