There is no law requiring 14th month pay. Hence, it all depends on what is stated on the agreement that the 14th mo pay is guaranteed. If it there are no discretion whether to give it or not and that it is guaranteed and without conditions, then it is not a benefit but part of your salary. As such, it becomes demandable as a matter of right.
As 14th mo pay as a rule is not demandable, it is deemed a bonus (subject to exceptions). The question really depends on whether or not a bonus forms part of wages depends upon the circumstances and conditions for its payment. If it is additional compensation which the employer promised and agreed to give without any conditions imposed for its payment, such as success of business or greater production or output, then it is part of the wage. But if it is paid only if profits are realized or if a certain level of productivity is achieved, it cannot be considered part of the wage. Where it is not payable to all but only to some employees and only when their labor becomes more efficient or more productive, it is only an inducement for efficiency, a prize therefore, not a part of the wage.