The name of "Royal" is given because it has state rank and it is the King who is responsible for sanctioning and ordering the publication and compliance of the rule and the name of "Legislative" is given because it is a delegation from parliament.
However, when the rule is created by an autonomous government, it receives the name of "Legislative Decree" because the King only sanctions the Decrees of the central government (the autonomous community Legislative Decree is sanctioned by the President of the Autonomous Community in the name of the King[1]).
In this way, the Legislative Decree that is created will already be backed by Congress and will become part of the legal system with full rights.
If the Constitutional Court considers the subject matter is not within its competence, it may declare it totally or partially unconstitutional and expel it from the legal system.
For this reason, it is often considered that the Constitutional Courts are negative legislators, lacking the power to make laws but with the power to repeal all or portions of the unconstitutional laws/acts.