An AEP file serves primarily as an AE project blueprint that contains the instructions for building your composition rather than a finished movie, including timelines, multiple layer types, animation data like motion controls, 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.Since AEPanced AEP file handler relies on external links, After Effects may report “offline footage” whenever source files get moved, renamed, or omitted during a transfer, which is why the Collect Files feature (or manually assembling the AEP and all used media) is the normal way to send a project reliably, and if an AEP refuses to open in After Effects, hints like its source, companion files, Windows associations, or a quick read-only text-editor view can indicate whether it’s truly an AE project or an unrelated format.When an AEP seems to break on another PC, it’s almost always because it’s a reference-only blueprint that depends on external media, and After Effects stores absolute paths to footage, graphics, audio, and proxies, so when moved to a system where those paths differ or the files weren’t copied, AE opens the project but can’t find the assets, resulting in Missing/Offline Media until everything is reattached.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 acts as a compact database for your After Effects project, which is why it can store an entire motion-graphics setup without matching the size of your footage, capturing details about comps—their resolution, frame rate, duration, nesting, and background—along with every timeline layer and its transforms such as position, scale, rotation, opacity, blend modes, mattes, parenting, and timing, plus animation elements including keyframes, easing, motion-blur settings, markers, expressions, and full effect setups, as well as masks or roto shapes with their paths, feather, expansion, and animated points.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 project logic and the file paths of the sources, causing missing-media alerts if items are moved or renamed.