Elective Deferrals and Union Employees

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Must an employer who maintains a 403(b) plan that consists exclusively of salary deferrals allow collectively-bargained employees to participate. My understanding was that unlike qualified plans, they could not be excluded from participation. However, section 4.72.13.6.1 of the Section 403(b) audit guidelines says that they can be excluded. When did the rules change (or was it always that way??)