A word for a temporary job position?

Regarding a job position which:

• Does not seek to replace someone else;
• It's not about a new employee;
• The person is entitled to work as the Chief/Head of the Department with the command of the CEO, to see how this person will handle this position for a certain period of time until entitled permanently or not at all.

I don't want to use the term "temporary", nor "interim", which is used in times of turmoil.

Can you suggest a more appropriate term given the situation above?

Such a person is said to be Acting in the position.

An Acting Manager is someone with all the powers of a Manager but is not formally or permanently assigned to the position.

The word can be applied to any post or rank. It is used in the military for someone temporarily promoted in the field - for example to replace an incapacitated officer.

If the person may either later be removed from the post or formally assigned to it. In the latter case, the acting prefix is removed.

If it is a position where the employee is to be evaluated after a certain period of time, after which a decision is made as to their continued employment, then probationary is what you are looking for.

Probation:

In a workplace setting, probation (or probationary period) is a status given to new employees of a company or business. It is widely termed as the Probation Period of an employee. This status allows a supervisor or other company manager to evaluate closely the progress and skills of the newly hired worker, determine appropriate assignments, and monitor other aspects of the employee such as honesty, reliability, and interactions with co-workers, supervisors or customers.

(Wikipedia)