Based on investigations of thermally stimulated conduction (TSC) in Sn2P2S6 single crystals and the effect of the surrounding gas on the character of the thermally stimulated current, it is concluded that the degree of unipolarity of ferroelectrics-semiconductors and the screening conditions play a significant and sometimes determining role in the filling of local levels, manifested in the TSC curves. By combining the standard method of TSC with the method of TSC in the photoelectret regime based on the “intrinsic” effect of the field, it is possible to distinguish the surface and volume attachment levels. The greatest filling of surface levels at the ends of domains is obtained from TSC curves measured in a vacuum on single-domain samples with the pulling field oriented in the same direction as the internal field and the greatest filling of volume levels is obtained with minimum unipolarity of the crystal and the creation of depleted layers on its free surface by adsorption.