The Cold-Blooded beings in this game are either reptiles, such as C'tissians, or cave-adapted beings unused to temperature fluctuations (such as Zotz). They do not want to fight in the Cold.

Just for reference, everyone within Temperatures colder than 1 suffers additional Base Encumbrance. The first point beyond 1, 2, adds 0.5; this is trivial, unless the unit is mounted and has another half-point from their Armor-&-Shield Encumbrance being halved. 3 adds 1.5, plus the penalty from 2. Every 5 points of Cold Resistance on a unit keeps them comfy in another point of Coldness, so 5 Cold Resistance removes the penalty from 2, 10 Cold Resistance removes the penalty from both 2 and 3, and so on.

On top of the above, Cold-Blooded fellows suffer extra Base Encumbrance from Cold Scales, on top of the typical penalties for this stuff. Each point of Cold colder than 0 adds 2 points.

New to Dominions 6, Cold Resistance can relieve Cold-Blooded units in the Cold. 5 Points will reduce their additional penalty by 50%, while 10 Points will remove that extra penalty entirely. Almost all Cold-Blooded nations have some source of this resistance.

For reference, here's how Coldness increases the Encumbrance of regular troops, versus Cold-Blooded ones. (Neither have Cold Resistance!)

Temperature	Regular Fellas	Cold-Blooded Fellas
1	+0	+2
2	+0.5	+4.5
3	+2	+8