In the Western parts of Europe of the Middle Ages everyone was supposed to be a Catholic. There were exceptions, Jews for example. But they were “othered” and not considered to be “of the nation.” So when dissenting Christians formed alternaive organizations, gatherings which claimed to be “church” but weren’t Catholic, it was a big deal. Worthy of commentary, censure, and persecution. These little protestant communities were called sects.
The word comes from the Latin and means a road. So the road or way of the Quakers as contrasted with the way or road of the Shakers. The powers that be conjured a different meaning, from a slightly different Lation ethomoly, a sect was an offshoot. Sociologists such as Max Weber and Ernst Troeltsch begging a definition more allied with the powers choose to define sect as a small, purified and distinctive breakaway from the prevailing church. So in the English context the Jesuits were labelled a sect,
Thus in the contrast established by usage, the broad, traditional and inclusive main body of a religion, versus the sects, which were organized around some “particular” doctrine or practice. Clearly the principle of postponing baptism until the age of consent was a big enough issue to designate the Baptists as a sect.
But there was something else that marked the sect and that was cultural style. Amish were quaint clothing, Unitarians like their sermons to be smart.
The socialist movement was shaped by sociologists, and so the idea of sect, and the designation “sectarian” came to usage in among socialists. A socialist sect was identified by some peculiar ideas, teachings, and its own “strategic vision” that were not shared by other socialists. Its had its own founders, leaders, teachers, again distinct from other socialists. And like the religious communities called sects, the socialist sects each have their own cultural style, their own ways of presenting themselves.
The United States in 2017 has a broad multi tendency and pluralistic Left, with many cultural styles. The Democratic Socialists of America present themselves to be an organization that could encompass the majority of this Left around an open perspective that works for a society that is both democracy (of the people) and socialist (for the people..)