Low Luminosity Quasi Stellar Objects comprise key objects in the context of galaxy evolution scenarios. They may represent the transition population between luminous QSOs and less luminous local galaxies and Seyferts. By analyzing the ionizing source within a galaxy, we distinguish different excitation mechanisms in a nearby sample of low luminosity QSOs. The diagnostic diagrams are a well-known tool for the taxonomy of sources dominating by AGN/Seyfert, LINER and Starburst activity. A significant number of the LLQSOs sample members show strong star forming activity, despite the fact that they were initially selected to be Seyfert-1. Furthermore, four galaxies were detected with a double narrow component in their spectral profile, indicating the possible existence of "superwinds". The comparison of the diagnostic diagrams coming from two data sets observed with different instruments drove to a certain "shift" in the classification of the sources, so called "aperture effect". The different techniques that can be used in order to obtain spectroscopic observations apply different aperture sizes, and this issue can play a critical role in the classification of the observed objects. An extended study on simulating such galaxies results to a solid explanation of variations in the diagnostic diagrams. An interesting result is the impact of the aperture effect not only in the local universe but also in larger cosmological distances. The importance of the individual classification of the galaxy region (nucleus, bulge, disk), as well as its effective area are also going to be presented. These two parameters are ultimately related and are needed to understand how an instrument configuration affects the final galaxy classification.