"convenio franja, o di centro de trabajo"
The synonym allows the interpretation as 'work centre'

“Franja” means in this context specific or particular “category” o “group” of workers within a company. The relevant Spanish law here is, as I am sure you know because you quote it in your post, the Estatuto de los trabajadores (Spanish Workers´ Statute, translation used by the International Labour Organization). However, the problem is that this Spanish law does not mention itself the term “franja” in particular, but the cumbersome expression “group of workers with a particular professional profile” instead. This is probably what makes the rendering a bit difficult at first sight (see explanation below).
An example for this kind of bargaining would be a collective agreement restricted to pilots of the Spanish national airline IBERIA, which would not affect the ground crews, but exclusively the group of pilots. Another example could be the collective agreements for engine drivers of RENFE.

The Estatuto does not mention, as stated before, the term “franja” and the reason for that is very simple: The “convenios franja or de franja” are not regulated by the Spanish law, but by the Spanish case law (= doctrina jurisprudencial) instead:
https://www.boe.es/buscar/act.php?id=BOE-A-2015-11430
ESTATUTO DE LOS TRABAJADORES
Artículo 87. Legitimación.
(…)
En los convenios dirigidos a un grupo de trabajadores con perfil profesional específico, estarán legitimadas para negociar las secciones sindicales que hayan sido designadas mayoritariamente por sus representados a través de votación personal, libre, directa y secreta.


The following useful link provides a thorough analysis of the so-called “convenios de franja”, both from the international as well as the Spanish perspective:
https://rua.ua.es/dspace/bitstream/10045/48227/1/Unit 3, Col...
UNIT 3: COLLECTIVE BARGAINING Dr. David Montoya
1.- Definition and function of collective agreements.
1.1.- Definition of Collective Agreement.
(…)
5.- “Estatutario” collective agreements.
5.1.- Negotiation units and scope of the collective agreement.
- Finally, the personal scope implies the group of employers and employees who will be affected by the collective agreement. It can be applied to all employees of the functional and territorial scope of the collective agreement or, instead, be just restricted to certain groups or categories of workers. In fact, the latter has been supported by case law. In this case what we have is the so-called “collective agreement for a group” or, in Spanish, “convenios colectivos de grupo o convenios franja” (e.g. collective agreement negotiated only for airline pilots).


http://www.fudepa.org/FudepaWEB/Actividades/FMedia/Instrumen...
Ámbito personal
Define a los trabajadores afectados por el convenio colectivo. El convenio suele aplicarse a todo el personal pero caben convenios que se apliquen a ciertas categorías o franjas de trabajadores.
En cuanto al ámbito personal, podemos distinguir entre los convenios que afectan a todos los trabajadores de un ámbito funcional y territorial, y aquellos otros que solo afectan a determinados colectivos que tienen capacidad para negociar su propio convenio y defender sus intereses particulares (convenio franja).


The International Labour Organization calls this type of agreements “collective agreements of branch”, a certainly shorter rendering, but to me not convincing.
https://www.ilo.org/wcmsp5/groups/public/---ed_dialogue/---d...
Collective agreements
(…)
However, Constitutional Court admits the validity of the exclusion of some categories of employees, e.g. company’s top executives. On the other hand, the so-called collective agreements of branch (“de franja”) are those which regulate the employment relations in certain profession (aviation pilots, train drivers...)