The differentiation of the different capacity types in Germany is quite confusing.
Here is an indication, what they are about (the precis conditions are ruled within the specific terms and conditions of the respective TSOs):
FZK: Frei Zuordenbare Kapazität - Freely Allocable Capacity: This is the usual term to firm capacity, allowing access to the Virtual Trading Point of the Entry-/ Exit-System. No condition is related to the use of the capacity.
bFZK: beschränkt Frei Zuordenbare Kapazität: conditionally Freely Allocable Capacity: In general, this capacity is also freely allocable and allows access to the VTP. Anyhow, the access to the VTP is subject to predefined conditions, e.g. temperature and pressure. If the conditions are not met, the corresponding Entry- or Exit-Points are limited, meaning wtih a specific Entry you can only reach a specified Exit-Point. Otherwise, the capacity might be interrupted.
DZK: Dynamisch Zuordenbare Kapazitäten: Dynamically Allocable Capacity: This capacity type allows only for restricted access to the VTP. On a firm basis, only predefined (Exit) Points can be reached with the assigned (Entry) Capacity. Thus, the VTP is not reached on a firm basis. If not assigend to a specific point, the VTP is accessible on an interruptible basis only.
BZK: Beschränkt Zuordenbare Kapazität: Restricted Allocable Capacity: Those capacities are only allowing the transportation from a dedicated Entry- to a pre-defined Exit-Point. They don't allow for access to the VTP.
Please be aware, that currently a concultation of BNetzA is ongoing (see: https://www.bundesnetzagentur.de/DE/Service-Funktionen/Beschlusskammern/1_GZ/BK7-GZ/2018/2018-0001bis0999/2018_0001bis0099/BK7-18-0052/BK7-18-0052_Verfahrenseinleitung.html ). Thereby it seems, whether BZK won't be allowed to be offered in the future, if BNetzA makes the decision as indicated.
In the consultation document of BNetzA, you also find a definition of the capacity types. It might help understanding: