An AEP file represents an AE project definition that outlines how your video is built rather than producing a playable export, capturing compositions, Layer elements of all types, animation data such as keyframes, effect setups, masks, mattes, plus cameras and lights in 3D space, and since it usually references media instead of embedding it, the AEP stays compact even when the project draws on large external assets.

Because AEP projects depend on external file paths, moving or renaming sources—or copying only the AEP to another system—can trigger "media missing" errors, making the Collect Files workflow (or a manual folder gather) the usual method to keep everything linked, and if an AEP doesn’t open correctly, factors like where it came from, what files accompany it, what Windows says under "Opens with," or a brief text-editor inspection can help identify whether it’s an authentic AE project or a separate vendor’s format.

When an AEP seems to stop working on a different PC, the cause is almost always that it functions as a reference-based blueprint instead of a self-contained package, with After Effects saving absolute file paths to video, images, audio, and proxy files, and when the project lands on a machine where those paths don’t match due to new drive letters, folder differences, or absent assets, AE loads the project but reports Missing/Offline Media until you reconnect the files.

A project may look misconfigured even with footage intact when the new machine lacks specific fonts—forcing text to substitute—or missing plugins that leave effects unreadable, or when using an older After Effects version that can’t interpret newer features, and the dependable solution is to use Collect Files or replicate the folder layout exactly and then relink, at which point matching fonts, plugins, and paths generally restore the project immediately.

An AEP file is a compressed structural database for AE so it can store an entire motion-graphics workflow while staying tiny, preserving comp settings—resolution, fps, duration, background, nesting—and all layers with transforms such as coordinate data, scale, rotation, opacity, blend modes, mattes, parenting, timing, plus everything related to animation: keyframes, easing, motion blur, markers, and expressions, along with the full effect chain and mask/roto elements including outline curve data, feather, expansion, and animated vertices.

With 3D enabled, the AEP records camera rigs, lighting setups, 3D layer options, and render configurations, plus project-organization elements such as bins, label colors, interpretation settings, and occasional proxy assignments, but not the actual footage files—your videos, images, and audio stay external—meaning the AEP is mostly the construction plan and the references to media, so if you relocate assets, After Effects reports missing items until you relink.