Much like the parable about blind men each characterizing an elephant by feeling different parts, we have identified different ways that faults can slip slowly by using different observation techniques. At the surface, this is known as fault creep. Immediately after an earthquake, this is known as post seismic slip, or afterslip. In the interval between earthquakes, geodetic networks capture episodic slow slip events classified by varying size, duration, and magnitude.
In a Commentary, Jolivet and Frank  suggest that these different classifications are artifacts from how and when they were discovered. They instead point out that there is no evidence for different physical causes of slow slip. They note that, much like their more obvious earthquake cousins, the slow slip events we observe happen on all temporal and spatial scales, and are part of the same intermittent, clustered process.
Citation: Jolivet, R. & Frank, W. . The transient and intermittent nature of slow slip. AGU Advances, 1, e2019AV000126. https://doi.org/10.1029/2019AV000126
—Tom Parsons, Editor, AGU Advances