The phrase "trajectory classes" refers to different groups or categories that objects or ideas can be grouped into based on their paths or directions of change over time. Full definition
Three distinct trajectory classes were identified: low stable, increasing, and decreasing. (link.springer.com)
Five distinct trajectory classes were identified: low - stable, medium - stable, decreasing, increasing, and high - stable. (content.iospress.com)
We then associated latent trajectory classes of aggressive / disruptive behavior across the two time periods using a transition model for the corresponding latent class variables. (link.springer.com)