A tool you can extend is a platform. One you cannot is a decision re-justified every project.
The SDK is TrackPlanner CAD’s, not the suite’s. TrackPlanner CAD has a modular architecture: plugins are loaded at start-up, some shipped with the product, others written by the customer against an SDK that is included in the TrackPlanner CAD licence.

What plugins are used for.
These are not hypothetical. They are the ones in use.
Validating the schematic plan
Project-specific rules beyond the validator built into TrackPlanner CAD.
Importing from other sources
Legacy databases, spreadsheets, other design tools.
Exporting to custom formats
Including the certified railML 3.1 export, itself a plugin.
Integrating external systems
Where the toolchain has to talk to something already in place.
Generating configuration files
The application configuration a specific platform expects.
Producing verification reports
The evidence a review or an assessor asks for.
In the TrackPlanner CAD licence.
The SDK comes with TrackPlanner CAD and gives access to all project data, so a customer can build their own plugin without negotiating a separate agreement first. TrackPlanner CAD →
Or we build it.
Customers who would rather have a plugin built than build one can have NEAT engineering services build it.
