CC Madhya 20.174
varṇa-mātra-bheda, saba — kṛṣṇera samāna
“The first manifestation of the vaibhava feature of Kṛṣṇa is Śrī Balarāmajī. Śrī Balarāma and Kṛṣṇa have different bodily colors, but otherwise Śrī Balarāma is equal to Kṛṣṇa in all respects.
To understand the difference between svayaṁ-rūpa, tad-ekātma-rūpa, āveśa, prābhava and vaibhava, Śrīla Bhaktivinoda Ṭhākura has given the following description. In the beginning, Kṛṣṇa has three bodily features: (1) svayaṁ-rūpa, as a cowherd boy in Vṛndāvana; (2) tad-ekātma-rūpa, which is divided into svāṁśaka and vilāsa; and (3) āveśa-rūpa. The svāṁśaka, or expansions of the personal potency, are (1) Kāraṇodakaśāyī, Garbhodakaśāyī, Kṣīrodakaśāyī and (2) incarnations such as the fish, tortoise, boar and Nṛsiṁha. The vilāsa-rūpa has a prābhava division, including Vāsudeva, Saṅkarṣaṇa, Pradyumna and Aniruddha. There is also a vaibhava division, in which there are twenty-four forms, including the second Vāsudeva, Saṅkarṣaṇa, Pradyumna and Aniruddha. For each of these, there are three forms; therefore there are twelve forms altogether. These twelve forms constitute the predominant names for the twelve months of the year as well as the twelve tilaka marks on the body. Each of the four Personalities of Godhead expands into two other forms; thus there are eight forms, such as Puruṣottama, Acyuta, etc. The four forms (Vāsudeva, etc.), the twelve (Keśava, etc.), and the eight (Puruṣottama, etc.) all together constitute twenty-four forms. The forms are differently named in accordance with the placement of the weapons They hold in Their four hands.