An AEP file generally functions as an Adobe After Effects project that contains the instructions for building your composition rather than a finished movie, including timelines, multiple layer types, animation data like expression-driven changes, effect parameters, masks, mattes, and 3D components such as cameras and lights, while referencing external media files to stay lightweight even if the project uses gigabytes of footage.

After Effects shows "cannot find source" when an AEP’s linked assets are moved or excluded during transfer, which is why proper relocation usually involves Collect Files or manually assembling the AEP and every referenced element into one package, and if an AEP doesn’t behave like an AE file, clues like its download source, neighboring files, Windows associations, or a read-only glance in a text editor can confirm whether it’s a real After Effects project or a different type entirely.

When an AEP seems to lose its media 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.

Here's more on AEP file recovery have a look at the web-site. Sometimes a project appears not functioning right even though the footage is there if the new PC is missing fonts—triggering text substitution—or lacks third-party plugins, disabling certain effects, or if a newer AEP is opened in an older AE version, and the proven fix is transferring via Collect Files or copying the entire folder tree, then relinking so that once fonts, plugins, and media paths line up, the project typically un-breaks right away.

An AEP file works as a compact blueprint-style database that captures your entire After Effects project without containing the heavy media, storing comp properties like resolution, frame rate, duration, nesting, and background color, every timeline layer and its transforms such as position, scale, rotation, opacity, blending, mattes, parenting, timing, plus all animation instructions like keyframes, easing curves, motion blur, markers, and expressions, along with complete effect configurations and any mask or roto data including Bezier paths, feather, expansion, and animated vertices.

When you use 3D tools, an AEP stores your camera setups, lighting, all 3D-layer attributes, and any render settings tied to them, along with project-organization info like folders, label colors, interpretation rules, and sometimes proxy links, but it generally doesn’t embed media—MP4s, MOVs, images, and WAVs remain separate—so the AEP holds the assembly instructions and the location references of the sources, causing missing-media alerts if items are moved or renamed.