The european legislation on social security premiums for aircrew requires that either a base is nominated for the crew member in question, self-employed or not (in which country you are then liable for social security premiums), OR the crew member is designated as having a flexible base.
In the latter case it does matter again if you are self-employed or not. The difficulty is that the country where you pursue your activity decides on your being self-employed or not based on the local criteria for self-employment as stipulated in the local social security legislation.
It is needless to say that many agencies and a certain airline from Ireland gratefully take advantage of this complex situation by flexibly basing a lot of their crew.
More and more countries, however, have taken an interest in these constructions and are questioning pilots over their employment status...