Rules of thumb refer to simplifation heuristics used in dealing with uncertainty, situations where tracking behaviors can be onerous, or areas where one-size-fits-all approaches may not be successful. They can be a useful tool to reduce the cognitive load of complying with a new behavior.
For example, a person may find it easier to "eat out at restaurants only 4 times per month" rather than "limit monthly restaurant spending to $200." Similarly, avoiding eating certain types of foods, e.g. fried foods or high-calorie drinks, may be easier to recall and comply with than hitting a daily calorie goal.