My conclusion up front: this is such a rare term, based on both a Google search and a brief survey of my colleagues (engineers in the field of rotating machinery), that it would be worthwhile to ask the client exactly what is meant. Also the German instructions are typically terse and unclear: what exactly is meant by "beachten"? Stay in the range, stay out of the range, watch the range closely and use your professional judgment? Therefore a clarifying query would be entirely in order.

All that said, here's some examples:

I have one source for you that sounds like a similar application. This patent (DE 102010027698 A1) defines the "Sperrdrehzahl" as follows:
"Die Sperrdrehzahl NLK ist eine Motordrehzahl, die eine Wiederaufnahme von einem Kraftstoffversorgungs-Stoppzustand und einen Übergang zu einem Freilaufbetrieb ermöglicht, und leicht höher eingestellt wird als eine Freilaufdrehzahl."
In English:
"The barrier speed N LK is an engine speed that allows recovery from a fueling stop state and a transition to a free-running mode, and is set slightly higher than a free-running speed."

Another patent (EP 2713051 B1) has a different sense of what the term means. Here it is the speed at which a prime mover (in this case a turbine, not a diesel engine) rotates when its load has been removed (in this case a compressor). This does not seem to be the same usage as the previous patent.