Unidirectional because it is working with dc.
Probably.
A bidirectional TVS will limit voltages to the same extent in both directions. A unidirectional TVS will be a normal diode in the other direction, so will short out any negative voltage more than about 0.7 V.
In a dc application, the unidirectional TVS will help to keep negative voltages low enough to avoid damage, while a bidirectional TVS will do nothing for a reversed supply. On a 5 V supply, a reverse supply of -5V will not be affected by a bidirectional TVS but will be shorted out with a unidirectional TVS
However, there are two problems with reverse voltage and a unidirectional TVS. Firstly, the TVS will only limit the voltage to 0.7 V or so, and some semiconductors will be damaged by a reverse voltage of 0.7 V. Secondly, shorting out reverse voltages can have it's own problems, as it would be relying on fusing or current limiting in the supply, and it's very easy to get the situation where a large reverse current continues for a long time and does damage that way.
It may be necessary or desirable to have robust protection against reverse connection. That could be as simple as a diode in series. If there is protection against reverse connection, it's probably better to use a bidirectional TVS to avoid shorting out the supply if there is a reverse connection.