Alex,
There are various contracts knocking about. Check carefully, as different countries have different contracts.
Poland, Lithuanian, Bulgarian and Hungarian bases: local contracts, social security deducted at source.
Other bases: agency contracts. Arrange own tax and social security.
Long and short of it is: if you plan on using social system in these countries, not a bad deal. If not, avoid.
6/4 available at all bases, tends to work better at larger ones with more crews. Swap system in use online. No restrictions. Or not many......