Why is my Machining Allowance not connected to each other?

Machining Allowances are added to the structure to guarantee enough material for post processing steps after the manufacturing of the design. When creating Machining Allowances the Interface Tool is used. Depending on the application of the tool these two different results can be created:

The left picture shows the selection and creation two separate Interfaces. One face is selected and the Interface created and then the next face is selected and an Interface with the same settings is created. The creations steps are shown below:

If both faces are selected simultaneously and one Interface is created. The Machining Allowance is one connected volume.

The right picture shows the selection and creation of the Interface while using the Offset Distance option in the Interface Tool. Here only one face is selected and with the Offset Distance Option, the front face is also considered as an Interface and one connected Machining Allowance is created.