The term pipeline is used within SPIP in the UNIX sense of the word. The pipeline executes a series of functions for which the result of one such function is used as input for the next. In this way, each function in a pipeline can use the data that are passed to it, modify them, use them, and return them. The results then act as arguments for the next function, and the next and so on until the last such function.
When calling a pipeline, the first function is very often passed data, or at least a default value. The results of the chaining of the various functions is then used or displayed depending on the situation at hand.
Certain particular calls on pipelines in SPIP are to be considered as triggers, in the sense that they simply declare an event, but do not expect any result to be returned from the various functions that the pipeline will call. Most of these triggers have a name that uses the prefix trig_.