When working as a freelancer, in most cases, it is mandatory to be registered as self-employed, or more specifically, under the Special Regime for Self-Employed Workers (RETA). This is required when you carry out an economic activity on a regular, personal, and direct basis for profit.
The confusion usually arises with the word “regular.” You are not required to register as self-employed only when the work is occasional. That is, this is an exception and only applies in specific cases where the activity is not carried out continuously or habitually.
For example, if you’re just starting as a freelancer or haven’t yet decided to take the full leap but want to try taking on a project or two, you don’t need to register as self-employed for a one-time job. This applies only when the activity is not habitual but is still for profit.
(For more information: Social Security: Registration)