You gain back your Constitution Modifier (provided it isn't negative) each time you take a full rest, provided you are not sick, encumbered, poisoned, or already fatigued. You may gain back your Constitution Modifier while suffering from these things if you take medicine (sickness), remove your burdens (take off armor, set aside your stuff), receive first aid (poison) or consume a full meal (two rations rather than one, or a proper meal and a drink).
If you are able to consume a nicely cooked meal and are not suffering from one of those above mentioned penalties, you instead roll your class Hit Dice rather than gaining back your modifier. If you are in civilization and can spend money on food, medical aid and debauchery, you roll a number of HD equal to your level each night for recovery.
At no time, short of magical healing, can you recover more HP than your Constitution attribute statistic.
|Rations need to be important, dammit.|
What does this mean?
A tough and hardy person of strong constitution can go without, while those who lack such a benefit always need to make camp and eat each night to recover their class HD as a recovery mechanism. This incentivizes keeping track of rations, cooking meals, making camp a real event each night, and also makes you consider putting yourself in a more vulnerable state so that you might recover more fully.
It also makes lower level parties will enjoy their return to civilization and focus on camping and cooking to survive while in the dangerous wilds; while higher level parties can rely more on magical healing but are still bound to mortal things like eating a meal and sleeping to feel better.
This makes supping from a "healing potion" or making deals with magic types for aid all the more likely.
What constitutes a nicely cooked meal?
Any rations you have to roll on a table that sounds nice. Anything you cook at camp. Anything with spices. Any meal you can share with another and still be full. Maybe a favorite food always counts as "nicely cooked" for these purposes, unless it is rotten. We'll work on it.
For Dogs, Familiars, and Animal Companions
So long as they're kept by the warmth of the campfire and given something to eat, they always roll their HD for recovery purposes.
Next time: Still hard at work on writing things for travel rules.