The homologation of qualification and legalisation are two different procedures with different legal effects.
Legalisation is when the authorities in your country certify that the degree is valid in Spain and issued by the competent authority. Therefore, the procedures required for legalisation and the university degree to be valid in Spain vary depending on the country issuing the university degree. In most countries, this process is identified by the Hague Apostille.
More information on the procedure for the legalisation of academic documents
Homologation is when the Spanish Ministry of Education equates your degree with the corresponding Spanish one to carry out your profession in Spain. This process can only be carried out at the Ministry.
Those who indicate on the pre-enrolment form that they have the homologation of the qualification must prove it by attaching the resolution of homologation of the Spanish Ministry of Education. Failure to do so may result in exclusion from the pre-enrolment process.
More information on accreditation by the relevant Ministry
Example of a recognised qualification (Royal Decree 285/2004)
Example of a recognised Spanish qualification (Royal Decree 967/2014)
Equivalence to qualification is a procedure by which the foreign qualification is granted, throughout the national territory, the same effects as the qualifications in the specific area and field of training to which the equivalence has been declared, with the exclusion of the professional impact concerning those qualifications that can be obtained by homologation.
The equivalence at the academic level grants the foreign qualification throughout the national territory, the effects corresponding to the academic level for which the correspondence has been declared.
Example of equivalence to degree and official university academic level (Royal Decree 967/2014)