In the opinion of the karmīs (fruitive workers), offering the results of karma is called servitorship. But according to Vaiṣṇava ācāryas like Rūpa Gosvāmī, servitorship means constant engagement in some kind of service to the Lord.
In the Skanda Purāṇa it is said that those who are attached to ritualistic activities, the four orders of social life and the four orders of spiritual life, are considered devotees. But when devotees are actually engaged in offering service to the Lord directly, these must be bhāgavatas, or pure devotees. Those who are engaged in fruitive activities, or prescribed duties according to the four orders of social and spiritual life, are not actually pure devotees. But still, because they are offering the result to the Lord, they are accepted as devotees. When one has no such desire, but acts spontaneously out of love of God, such a person must be accepted as a pure devotee. The conditioned souls who have come into contact with the material world are all more or less desirous of lording it over material nature. The system of varṇāśrama and the prescribed duties under this system are so designed that the conditioned soul may enjoy in the material world according to his desire for sense gratification and at the same time gradually become elevated to spiritual understanding. Under these prescribed duties of varṇa and āśrama there are many activities which belong to devotional service in Kṛṣṇa consciousness. Those devotees who are householders accept Vedic ritualistic performances as well as the prescribed duties of devotional service, because both are meant for satisfying Kṛṣṇa. When householder devotees perform some Vedic ritualistic duties, they do so to satisfy Kṛṣṇa. As we have previously discussed, any activity aiming at satisfying the Supreme Personality of Godhead is considered devotional service.
Śrīla Rūpa Gosvāmī describes one who is fit for becoming engaged in devotional service. He says that persons who are neophytes and who have developed a little love of Godhead are not interested in the activities of sense gratification, in proportion to their devotion. But if there is still some attraction for sense gratifying activities, then the result of such activities should be offered to Kṛṣṇa. This is also called engagement in the service of the Lord, with the Lord as the master and the worker as the servant.
In the Nāradīya Purāṇa there is a statement of how this servitorship is transcendental. It is said there that a person who is constantly engaged in devotional service by his body, mind and words, or even a person who is not practically engaged but is simply desiring to be so, is considered to be liberated.