By default, the system is configured to prevent clashes between all service types.
You can view the configuration of these rules by doing the following:
Click Settings.
Hover over Planning.
Click Settings.
Click into Start of System.
Select the Clash Tab.
By setting the Clash Full Status to Enforce, the system won't allow for clashing at all, while setting it to Warn gives a warning whenever there's a clash.
The colour of the visit on the Planning screen will also turn amber to warn that there's a clash.
This handy guide provides more information about how to ignore the clash rules for specific service types.
If you want to amend the clash rules for all service types, please raise a new case online referencing this article.