LDK vs. SLU

Both the License Development Kit and the Server Licensing Utility perform the same function of registering the Runtime license for the system on which it will be run. Both are provided in order to give you some options.

If you are deploying to your own systems (e.g., for enterprise IT staff), the use of an Accusoft branded Server Licensing Utility may meet your needs. Similarly, if you handle the deployment onto end users systems, you may also find the Server Licensing Utility to be sufficient.
If you wish to hide this licensing registration from end users, you will likely choose the License Development Kit, which allows you to integrate the registration process into your application. The License Development Kit also allows for the optional assignment of Access Keys to specific users, which lets you manage your licenses more directly and limits the distribution of licenses.