A rectangular source cannot both remain complete and fill a square without padding, crop or distortion. FavBundle never stretches it, so contain and cover expose the real tradeoff.
Contain preserves every edge
Contain scales by the smaller axis ratio and centres the result. Empty space receives the selected opaque background. It suits a wide wordmark that must remain complete, but the meaningful mark may become very small at 16 pixels and should be inspected rather than assumed legible.
Cover fills by centre crop
Cover takes the largest centred square from the source and maps it to every output. It removes equal amounts from opposite sides and works for a centred symbol with disposable surroundings. It can cut text or an off-centre detail; preview every size and return to the source artwork when a dedicated small mark is needed.
Verify in the destination
Place the files at the paths named by the snippet, serve the manifest with an appropriate content type, clear favicon caches, and inspect real browser tabs, bookmarks and install surfaces. Keep the source artwork and compare small edges, contrast and crop. A structurally valid package proves its own bytes and names, not that every browser or operating system will choose the same asset or display it identically.