JSON hash and CSS output
The JSON download uses a TexturePacker-style “hash” shape: a meta block with the atlas size, and a frames object keyed by the file name (without extension). Each frame has x, y, w, h. rotated and trimmed are always false on the free packer. Many Phaser, PixiJS and Godot workflows can read this shape, or can be mapped to it with a short script.
The CSS download writes a .sprite base class that points at sheetpack.png, plus one class per frame with width, height and background-position. Rename the image in the CSS if you keep the atlas under a different file name.