ATKD Editing
ATKD stands for "attack data" and is the filetype "magic" for attack_data.bin files, located in a character's AI folder. Smash 4 AI do not calculate collision between hitboxes and hurtboxes at runtime, so they rely on this filetype to define a the rectangular "danger zone" of each move.
Each entry in the ATKD file consists of only 7 parameters, in order: Attack ID, First frame, Last frame, X1, X2, Y1, Y2. When changing movesets, it is important to also adjust the ATKD for any radical alterations, or else the AI will not know how to respond when being attacked by the move, nor will it be able to use the move at the correct ranges.
ATKD is essentially a very rough solution to the AI's problem of dealing with hitbox sizes. The range does not change depending on the frame of the move, it only considers rectangular areas, and has no notion of hitbox strength/sweetspots/etc.
Smash Forge's ATKD Editor allows either editing an ATKD by itself or editing the file in conjunction with a character's loaded moveset. It's highly recommended to use the latter in order to efficiently work with ATKD. In order to load the ATKD Editor with a character's moveset, simply enable the "Load/Render ATKD" checkbox in Render Settings (under "Hit/Hurt boxes" tab), then use Open Character on the desired character.
The Model Viewport and ATKD Editor will load under separate tabs, and from here you can take advantage of several conveniences. When you select an animation you will notice the currently selected cell in the ATKD Editor shifts to the entry of the corresponding attack ID (if one exists).
When you change the animation frame to one in which the ATKD specifies the attack should be active, the rectangle defined by the ATKD entry will be drawn in the viewport. In order to edit X and Y boundaries you can either do so in the ATKD Editor cells or click and drag the rectangle within the viewport itself.
The ATKD will save to the character's data by default when you press Ctrl + S in the ATKD Editor tab, but you can change the save output to another file with "Save As". When the ATKD Editor is closed, the render will close as well, and you will have to open the character again in order to open the editor again.