PD07 and PD10 touch on purpose; in a roundabout way you could derive a definition of limits from them. Not the word, but the usefulness of the idea.
QuotePD7: Some people want to be well esteemed and widely admired, believing that in this way they will be safe from others; if the life of such people is secure then they have gained its natural benefit, but if not then they have not gained what they sought from the beginning in accordance with what is naturally appropriate.
QuotePD10: If the things that produce the delights of those who are decadent washed away the mind's fears about astronomical phenomena and death and suffering, and furthermore if they taught us the limits of our pains and desires, then we would have no complaints against them, since they would be filled with every joy and would contain not a single pain or distress (and that's what is bad).