Toughness (being resistant) is a skill in EXODE. It's main use is to determine health value.

Base health is:

Toughness * 20 (when the game uses the “10 display scale”, with base scores from 1 to 13, similar to Quality Level)

Or:

Toughness * 2 (when the games uses the “100 internal scale”, with base scores from 1 to 130)

If a character is highly resistant, you will see the keyword “Tough” appear in their file.

Toughness is also used is a huge number of situations such as resisting to disease or other environmental factors.