A supplier's product photo and a portrait can share an upload box without sharing the same ratio. Fitting keeps the entire picture and calculates a size that stays inside the specified limits.
Use Fit Image Inside Box when width and height describe maximum dimensions rather than an exact canvas. This tool does not add padding to make every result the same rectangle.
The limiting side decides the result
A 1200 by 800 photo fitted inside 600 by 600 becomes 600 by 400. The width reaches the limit first, and the height stays smaller so the ratio is preserved.
A 300 by 200 source with the same box remains 300 by 200 when enlargement is disabled. Turn on enlargement only if you accept that a small source will be resampled upward.
That distinction is useful for galleries where images must stay under a ceiling but should not be enlarged unnecessarily. It also avoids mistaking a maximum size instruction for an exact dimension requirement.
The output is not a padded square
If a destination insists on 600 by 600, a 600 by 400 result will not satisfy that exact rule. You need a square crop or a canvas that includes the whole image with empty space around it.
Cropping removes content. Padding keeps it. Decide which treatment is suitable for the subject before selecting another tool.
The same maximum box can be applied to several source images. Each result is calculated separately, so their final dimensions can differ.
Check the dimensions shown under each download, especially in a mixed batch. An image that fits your limits can still exceed an upload byte allowance, and changing the format or compression may be necessary as a second operation.