Fit the whole photo, or fill the frame
Fit inside
The whole photo is kept and placed in the middle. The space around it is filled with a soft, blurred copy of the same photo, so a landscape shot looks at home in a tall story instead of sitting between black bars. White, black or a transparent background are there too.
Crop to fill
The photo fills the whole frame and the edges that don’t fit are trimmed equally from both sides. Use it when the subject is in the middle and the edges don’t matter.