To run a simulation, not only must the circuit be defined, but also the type of analysis to be performed. There are six different types of analyses: linearized small signal AC, DC sweep, noise, DC operating point, small signal DC transfer function and transient analysis. Precisely one of these six analyses must be specified.
Whereas the circuit topology is typically schematically drafted, the commands are usually placed on the schematic as text. All such commands start with a period and are therefore called "dot commands".