Skull is poison
There's also a QR code-liked icon means quickhacking, but not a percentage but a number, the number is the basic consumption of RAM, it also shows the degree of defense/weakness to quickhacking damage
I don't know how to calculate the percentage, anyway the higher the percentage, the higher the weakness/resistance.
These indicate weaknesses against various attributes, as well as the red "resist to". The average resistance of mobs seem to be 15 and weakness seem to be 30. Boss usually have higher resistance and quickhacking defense.
Different affiliations weaknesses/resistances are also different. Take quickhacking defense as an example:
Very low - Scavengers, 6th Streets, Wraiths
Low - Animals, Valentinos
Medium - Maelstrom
High - Tyger Claws
Very High - Voodoo Boys
Corpos are usually around medium, netrunners are always higher than other units of the same affiliation. NCPD are immune to quickhacking, unless special situation
The attributes weakness/resistance is also related to affiliation. For example, Wraiths have 40 poison resistance (even higher than some bosses). Kang Tao have non-fixed resistance. Robots are always weak to EMP and resist to fire and poison. "No affiliation" are usually medium attributes and quickhacking resistance.