When working at a large organization with many divisions or departments, it may be necessary to differentiate between the extent of your working relationship with people in your organization. Co-workers would be those who work in your workgroup or department, or those whom you have a close working relationship with in another workgroup or department. Workplace acquaintances are people you know of but don't have a close working relationship with. These individuals are typically in other departments or divisions of the company.
In office fraternization situations, workplace acquaintances can be considered (although weakly so) the exception to the "I don't date co-workers" rule. The argument being that because he or she is not considered a co-worker (defined in the preceding paragraph), that person can be considered dating material because, even though you both work in the same company, you both work in a different division or department.
Rob: I am thinking of asking Trish from Accounting out to dinner and a movie.
Jon: Cool, she's hot! But you said you don't date co-workers.
Rob: She's in a different department, so that doesn't make her a co-worker, just a workplace acquaintance.