We don't like this terminology either, but with regard to labor relations, it is a legal designation specified by section 5 USC 7103(15) of the Federal Service Labor-Management Relations Statute or the Statute:
"professional employee" means-
(A) an employee engaged in the performance of work-
(i) requiring knowledge of an advanced type in a field of science or learning customarily acquired by a prolonged course of specialized intellectual instruction and study in an institution of higher learning or a hospital (as distinguished from knowledge acquired by a general academic education, or from an apprenticeship, or from training in the performance of routine mental, manual, mechanical, or physical activities);
(ii) requiring the consistent exercise of discretion and judgment in its performance;
(iii) which is predominantly intellectual and varied in character (as distinguished from routine mental, manual, mechanical, or physical work); and
(iv) which is of such character that the output produced or the result accomplished by such work cannot be standardized in relation to a given period of time; or
(B) an employee who has completed the courses of specialized intellectual instruction and study described in subparagraph (A)(i) of this paragraph and is performing related work under appropriate direction or guidance to qualify the employee as a professional employee described in subparagraph (A) of this paragraph
The AFGE has provided a list of job titles that are considered non-professionals. We assume most others job series would fall into the professional category. We have also compiled a list of position categories based on various voter eligibility lists.
From what we can tell, the professional category seems to include those positions listed on the OPM General Schedule Qualification Standards - Standards by Occupational Series, that indicate "Min. Ed." or "License or Certificate".
As with other activities, use common sense. You should not perform union activities during your work time or with the use of government equipment. However, "your employer cannot prohibit you from talking about or soliciting for a union during non-work time" (e.g., during breaks).
For additional information about Your Rights during Union Organizing activities, visit the National Labor Relations Board website.