An AET file is typically used as a reusable AE template, acting like a master version of an AEP that you open to create fresh projects without touching the original, and inside it holds the blueprint for the animation such as compositions, timelines, layered elements, keyframes, effects, expressions, cameras, lights, global settings, and the project’s internal organization including folders and interpretation rules.

An AET generally doesn’t embed media, instead storing references or paths to external audio, images, and video, which is why template sets are bundled as ZIPs with Footage/assets folders and why After Effects reports missing items if things were moved, and because AETs might rely on certain fonts or plugins, opening them elsewhere can trigger alerts until everything is installed, and since file extensions aren’t exclusive, checking "Opens with" or the file’s source location is the best way to confirm the program behind it and what extra files should accompany it.

An AEP file serves as the primary file you update during animation, while an AET is a reusable template, so in practice the difference lies in purpose: you open an AEP to continue that same project, but you open an AET to start a fresh file so the original stays clean.

That’s why AET files are standard choices for motion-graphics template packs like intros, lower-thirds, and slideshows: the creator preserves the AET as the untouched master and you open it only to Save As a separate AEP for each new video, replacing text, images, colors, and logos, and even though both AET and AEP hold the same kinds of data—comps, layers, keyframes, effects, expressions, cameras/lights, and settings—and both normally reference external media, the AET’s job is to safeguard the template while the AEP becomes the project you actively modify.

An AET file mostly contains the framework and behavior of a motion-graphics project rather than the footage itself, offering compositions with resolution, frame rate, duration, and nesting, and keeping the entire timeline of text, shape, solid, adjustment, and precomp layers, with properties such as position, scale, rotation, opacity, masks, mattes, blending modes, parenting, and all animation elements including keyframes, easing curves, markers, and optional expressions.

Additionally, the template includes effects and their configurations, such as color correction, blurs, glows, distortions, and transitions, together with any 3D setup—cameras, lights, 3D layer settings—and render/preview preferences, plus project structure like folders, labels, interpretation settings, and proxies, though it usually omits embedding raw media, fonts, or plugins, depending on linked paths that can lead to missing-footage or missing-effect warnings when the file is opened elsewhere Here is more info regarding AET file opening software stop by the web site. .