A) What "free choice subjects" are:

https://educadamentesite.wordpress.com/2015/12/28/las-asigna...

B) The term "free choice subject" appears here:

http://www.spainexpat.com/spain.php/information/schools_and_...

and here (PDF file - page 8):

http://www.ub.edu/euelearning/icurriculum/Catalan ICT curric...

C) At the end of page 2 of the following form (PDF) you can find a distinction made between "free choice subjects" that are either compulsory or optional:

https://orientagala.files.wordpress.com/2016/09/2c2babachici...

Spanish term (edited): asignaturas de libre configuración autonómica -

The "bloque de asignaturas de libre configuración autonómica", as the term suggests, are subjects that the regional governments in Spain (known as "Autonomías" or "Comunidades Autónomas") can include in the curriculum of State and subsidised private schools within the framework of the national eduction plan. They include, for example, regional languages in some comunidades (Galicia, the Basque Country, Catalonia...) which, far from being elective, are compulsory in the regions that have them. The education authorities in the autonomous regions establish the curriculum in their respective areas of administration, which must include the minimum teaching requirements established by the State (50% of the scholl day in regions with 2 official languages and 60% elsewhere). The rest are "asignaturas de libre configuración autonómica".