A little info.:Monastic divisions
Among the Theravāda nations of Southeast Asia and Sri Lanka, nikāya is also used as the term for a monastic division or lineage; these groupings are also sometimes called "monastic fraternities" or "frateries". Nikāyas may emerge among monastic groupings as a result of royal or government patronage (such as the Dhammayuttika Nikāya of Thailand, due to the national origin of their ordination lineage (the Siyam Nikāya of Sri Lanka), because of differences in the interpretation of the monastic code, or due to other factors (such as the Amarapura Nikāya in Sri Lanka, which emerged as a reaction to caste restrictions within the Siyam Nikāya).
These divisions do not rise to the level of forming separate sects within the Theravāda tradition, because they do not typically follow different doctrines or monastic codes, nor do these divisions extend to the laity.http://www.answers.com/topic/nikaya