The Amarapura Nikaya is a Sri Lankan Theravada monastic fraternity founded to escape a caste restriction, and its origin is a direct consequence of the closed character of the older order.
Ordination refused, then imported
The valid higher ordination restored to Sri Lanka in the eighteenth century was held by the Siyam Nikaya, whose ordination was confined in practice to the highest, govigama, caste. Monks from other castes, excluded from that ordination, travelled to the Burmese royal capital of Amarapura in 1803 to receive higher ordination there and brought the lineage back to the island, founding a new fraternity named for the city that had ordained them. The founding of the order is therefore not a doctrinal dispute but a caste dispute, worked out through the acquisition of a rival ordination lineage from abroad.
A federation of lineages
The Amarapura Nikaya grew as a body drawing its monks from the communities the Siyam Nikaya would not ordain, and it developed as a looser federation of several lines rather than a single centralised order, reflecting its origin among different excluded groups. Together with the later Ramanna Nikaya it forms the counterpart to the caste-restricted Siyam Nikaya, so that the three-fold division of the Sri Lankan sangha is in large part a record of a social exclusion answered by the importing of ordination. The order remains one of the principal fraternities of Sri Lankan Buddhism, and it later played a significant part in movements of Buddhist revival on the island.