There can be multiple types of mounting styles for box fans. In this post, I will clarify the differences between two common types of fans.
The two I will be discussing are ribbed, and ribless/flange as shown in the photo below.
Mounting a ribbed fan, you will use hardware that goes through the whole length of the fan to be fastened on the other end. This type provides the structure needed for mounting to both sides of the fan.
Mounting a ribless fan, you will use hardware that only uses the flange on the side attached to the unit you are mounting it on.
Shown below it is not advised to use mounting hardware that uses both flanges of a ribless fan as it can cause stress to the unsupported flange as it will flex when fastened.
Source for images, https://us.misumi-ec.com/maker/misumi/el/tech/den/fan/gijyutu.html