Why bleed exists
Printers cut stacks of hundreds of sheets at once, and the blade drifts. If your art stops exactly at the trim line, a cut that lands a hair outside it leaves a sliver of white paper. Bleed is art that runs past the trim on purpose, so any drift still hits ink. The safe area works the other way: it keeps important content far enough inside the trim that a cut landing inside the line never removes it.
How the guides are placed
With the canvas origin at the top-left corner of the bleed box, the trim box starts at (inner bleed, top bleed) and the safe area starts at (inner bleed + inner safe, top bleed + top safe). Multiply by DPI for pixel offsets. Those are the offsets for a right-hand page, with the spine on the left. Where a spec treats the spine edge differently (KDP and IngramSpark have no bleed there), choose Left-hand page and the spine moves to the right. For a double-page spread the two trims sit side by side, only the outer bleed is added to the total width, and the fold is at outer bleed + one trim width from the left edge. Keep faces and lettering off the centre 0.5 in as well.
Typical values
0.125 in (3 mm) bleed is the near-universal default. Safe areas range from 0.25 in (PrintNinja) to 0.5 in (IngramSpark). Ka-Blam asks for 0.1875 in on the sides; IngramSpark wants no bleed on the bind edge. Details in how much bleed does a comic need? and comic safe areas explained.
Questions
How much bleed does a comic need?
0.125 inches (about 3 mm) on every edge is the standard almost every comic printer asks for. A few require more on the outside edges or none on the bind side.
What is the safe area?
An inset inside the trim, usually 0.25 to 0.5 inches, where lettering and important art must stay so a slightly off cut never clips them.