  Modern software systems are becoming increasingly complex and opaque. The
integration of explanations within software has shown the potential to address
this opacity and can make the system more understandable to end-users. As a
result, explainability has gained much traction as a non-functional requirement
of complex systems. Understanding what type of system requires what types of
explanations is necessary to facilitate the inclusion of explainability in
early software design processes. In order to specify explainability
requirements, an explainability taxonomy that applies to a variety of different
software types is needed. In this paper, we present the results of an online
survey with 84 participants. We asked the participants to state their questions
and confusions concerning their three most recently used software systems and
elicited both explicit and implicit explainability needs from their statements.
These needs were coded by three researchers. In total, we identified and
classified 315 explainability needs from the survey answers. Drawing from a
large pool of explainability needs and our coding procedure, we present two
major contributions of this work: 1) a taxonomy for explainability needs in
everyday software systems and 2) an overview of how the need for explanations
differs between different types of software systems.
