It means a service that can be employed, can be delivered in exchange of payment. Thus, for hire.

dependiente = you are under control

por cuenta ajena = somebody else owns the service you produce

an option

As opposed to an "independent service". As I understand it, the context here relates to the idea of false self-employment, so simply using "employee" here is insufficient because the term "employment" is broad enough to include both those employed by others, those who are correctly identified as "self-employed" and those who who fall somehow in between those two concepts, whether by accident or by design.

Economically dependent worker
11 June 2007Observatory: EurWORK

The concept of ‘economically dependent worker’ falls between the two established concepts of employment and self-employment. It refers to those workers who do not correspond to the traditional definition of employee because they do not have an employment contract as dependent employees. However, although formally ‘self-employed’, they are economically dependent on a single employer for their source of income.
https://www.eurofound.europa.eu/observatories/eurwork/indust...

Key differentiators here seem to be the concept of dependence or subordination, and whether or not the worker is free to use the methods they wish to achieve the outcome desired by the "employer". It's a complicated subject made more complex by employers' seeking to evade their responsibilities as such under
the various labour laws in place around the world, and I presume that's what the asker's text is about, i.e., precisely defining these circumstances in terms that are more precise than simply "employee".