Its really important not to confuse processes.
A disciplinary investigation is a standalone. It may have other processes that need to complete before the disciplinary investigation can be finalised but it does not merge with others.
For example, a case where there was potential gross misconduct resulting in an accident investigation. Through the disciplinary process, the employee may be suspended pending the outcome of the disciplinary investigation.
However the Accident investigation will need to be concluded and will likely be used as evidence in the disciplinary investigation.
The Accident Investigation is separate to the disciplinary process.
If you need to know more then do contact me.