When people of the world speak of adab (proper conduct), they generally understand it as eloquence, rhetoric, memorization of knowledge, entertaining rulers, poetry, and artistry.
The adab of the people of religion, however, is more about the discipline of the nafs (the lower self), training the body (bodily desires), purifying the qalb (the heart), safeguarding the boundaries of the sharia (the sacred law), abandoning shahwa (carnal desires), avoiding doubtful matters, devoting oneself to acts of obedience, and competing in goodness.
The understanding of adab among the elite (khawāṣṣ, spiritual elect) of the people of religion is more concerned with the purification of the qalb, safeguarding secrets, loyalty to covenants, beautiful adab in maintaining one's ḥāl (spiritual state) and moments, not giving undue importance to inspirations such as khawāṭir (passing thoughts), wārid (spiritual influx), and ilhām (inspiration), the unity of one's inner and outer worlds, and the beauty of adab in the stations of proximity (qurb), moments of seeking, and times of presence and union. (Lumaʿ, 195)
There are three things that a sincere faqir (poor one, i.e., Sufi ascetic) must observe regarding his faqr (spiritual poverty): not asking when in need, not refusing when given, and not holding onto what is received until a second time (i.e., to immediately part with it or give it as charity). (Lumaʿ, 231)
The adab of Sufi faqirs at the table is that they are not sorrowful or anxious during meals, do not act artificially, and prefer what is good, pure, and little over what is much and bad. They do not have a fixed time for eating. When food is ready, they do not consume mouthfuls in rapid succession. When food is offered to them, they do not refuse it. They dislike eating so much as to burden or harm the stomach. Even when very hungry, they always observe the proper adab of eating. (Lumaʿ, 243)
Adab is the safeguarding of secrets (one's inner world) from khawāṭir (passing thoughts), the preservation of moments, and distancing oneself from envy and enmity.
Ruwaim was asked about the adab of the traveler (adab al-musāfir, the proper conduct for one who wishes to set out on a journey). He replied: Since his heart is wherever his destination is, his foot does not outpace his thought. (Lumaʿ, 302)
Adab among people (ādāb fi'n-nās) is to preserve muruwwa (virtuous conduct, humanity, or character). Adab in religion is to preserve the Sunnah (the Prophetic tradition, i.e., to persist in following the Sunnah). Adab in love is to preserve respect (i.e., to observe or maintain reverence). These three forms of adab are interconnected, for one who lacks muruwwa does not preserve the Sunnah, and one who does not preserve the Sunnah does not observe respect.
The preservation of adab in dealings (and relationships) is in the qalb, in what is sought.