I think it is determined by 5 Data - lines.
Data - Leave Target Alive True/False
Data - Life Conversion As Percent True/False
Data - Life Converted to Life 0.00
Data - Life Converted to Mana 0.00
Data - Mana Conversion As Percent True/False
Data - Leave Target Alive
If it is ticked as True, the Target die. If it is False, the target live.
Data - Life Conversion As Percent True/False
Data - Life Converted to Life 0.00
"Data - Life Converted to Life" (I will call it LCTL) indicates the amount of Life will be stolen from the target. If "Data - Life Conversion As Percent" (LCAP) is set as False, the amount in the LCTL will be percentage as 1.00 equal 100% of the target current life. If the LCAP is set as True, the amount in LCTL will be taken as points.
Texting yields that
Data - Life Conversion As Percent True
Data - Life Converted to Life 50.00
The Target loses 50 Hit points
Data - Life Conversion As Percent False
Data - Life Converted to Life 0.50
The Target loses 50% of its current health.
When "Data - Leave Target Alive" (LTA) is set to True, the amount of health the target lose in LCTL is also dealt to the Caster.
Which means when LTA is True, If the target unit lose 50 hit points, your caster lose 50 hit points. If the Target unit lose 50% of its current health, your caster also lose the same amount.
My Paladin after casting Steal on Troll
Troll currently has 300 HP.
When LTA is True, the Troll loses 50% of its Health (=150 HP). My Paladin also loses 150 HP equal to the amount Troll loses
When LTA is False, Troll die. My Paladin is healed for 150 HP equal to 50% of the amount of Troll's current health.
The Steal is pretty much "Hit point Removal" type. My Paladin with Divine Shield on still loses life.
Data - Life Converted to Mana 0.00
Data - Mana Conversion As Percent True/False
These 2 lines, however, only affect the mana of the unit, not the Life as said in the Life Converted to Mana and it function with the same rule as the 2 above it.