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I am making the schematic of a 24 V power supply regulator using the LM74900-Q1 IC. When selecting the TVS diodes, I found the following design suggestion in the datasheet of another IC (LM74800-Q1) for 24 V battery systems:

Single bi-directional TVS cannot be used for 24-V battery protection because breakdown voltage for TVS+ ≥ 65V, maximum clamping voltage is ≤ 53 V and the clamping voltage cannot be less than the breakdown voltage. Two un-directional TVS connected back-back needs to be used at the input. For positive side TVS+, SMBJ58A with the breakdown voltage of 64.4 V (minimum), 67.8 (typical) is recommended. For the negative side TVS-, SMBJ28A with breakdown voltage close to 32 V (to withstand maximum reverse battery voltage –32 V) and maximum clamping voltage of 42.1 V is recommended.

I don't quite understand the part that says Two un-directional TVS connected back-back needs to be used at the input. I tried to look for pictures but I found that it depends on the type of diode.

What is this type of configuration? What does it look like? Are there any characteristics of placing them in this way? Does it increase the clamping voltage or the breakdown voltage?

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First, there's a typo in your quoted text. It's not un-directional it's uni-directional. Un-directional would have no direction which makes no sense, uni-directional works in one direction.

Connecting TVS diodes back to back like so:

schematic

simulate this circuit – Schematic created using CircuitLab

gives you the reverse breakdown voltage of a single device plus the forward drop of one device in each direction, so say they were 24 V diodes and had a forward drop of 0.7 V, each direction would be the equivalent of a 24.7 V diode.

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