When I create a boolean variable, automatically two limits are created. When I create a variable based on an USINT, no limit is created. Where can this be configured? Can the defaults be changed?
If a limit is created automatically at creating a variable, is specified at the datatype this variable is based on. The boolean datatype has two limits per default. It is of course possible to create a new user-defined datatype based on the IEC datatype BOOL, without any limits. (e.g. if a reaction matrix is to be used)

As only one limit can be active at the same time, logically the number of limits at the boolean datatype / variable is limited to two.

Also the standard IEC Datatype USINT can be modified to contain one or more limits.

Also other properties (defaults) at the variables are inherited from the datatype, however for some properties a link exists to the property of the datatype. (visible in the properties window of the variable by the arrow symbol). If the property is changed at the datatype, this is changed at all variables based on this type. Of course if a property is changed directly at the variable, the link to the datatype is cut. This link however can be restored in the context menu of the property itself.