Dear Col RK,
Companies hire consultants because they have expertise of some field. Many times their role is advisory and they may not have day to day job. That is why consultants visit their client on case to case basis. In certain cases consultants make visit every week, month etc.
Employer-employee relationship is essential relationship. In contrast, company's relationship with the consultant could be called as non-essential or extraneous. Therefore, question of allotting some designation does arise. Designations are given to the employees and not to the consultant. Designation of "Officer" is given to the paid employee and not to the consultants.
Therefore, when company enters into agreement with the consultant, the agreement may mention that person as "Consultant" or "Principal consultant". Very specific examples are "ISO Consultant", "Procurement Consultant", "Showroom Consultant" and so on. Consultants are service providers. To measure their performance, how the services will be measured can defined while framing the contract agreement. This brings clarity and avoids confusion at later stage.
Thanks,