Years ago we have compiled all the available optical data in the literature to build a comprehensive classification of the low-ionization structures (LIS), as knots, jets and jet-like structures in planetary nebulae (PNe). This classification highlighted a number of questions that 6 years later still remain open, such as, what is the role of magnetic fields and of the binary systems on the formation of collimated LIS, or, how do collimated low-velocity LIS survive throughout the PNe evolution? Other very puzzling properties were found in a few cases and need to be explained: i.e., the fact that highly supersonic LIS are mainly photoionized (e.g., in NGC 7662 and NGC 7009) or the lack of significant density contrasts found in collimated and isolated knots. In an attempt to provide an answer to the questions above we have undertaken a systematical exploration of the physical properties of the LIS. By using our own optical long-slit spectra for 20 PNe, we have estimated the electronic temperature and density of the LIS and compared them with those of the their surrounding nebula. Here we discuss our newly found correlations between the type of the LIS and its physical properties for NGC 7662.