There are two main classes of SPSS commands: Data reading/manipulation commands, and Analysis commands. The data reading and manipulation commands are used to convert raw data, or data entered in other types of software (e.g., dBase, Excel) into an SPSS Active File. The Analysis commands are used to analyze the data that has been prepared in the Active File.

It is crucial to understand that Analysis commands can only access (analyze) data that have been converted into an Active File, and that there can be only one Active File at any given point in an SPSS program (or SPSS interactive session). Another key point is that all data needed for an Analysis command must be in the active file at the same time. SPSS Analysis commands cannot access data in two different places. If necessary, system files (or combinations of system files and the Active File) must be merged into one Active File prior to running an Analysis command.