To Place a Transition Using the Join with Transition tool

Select the Join icon from the Manipulation toolbar. The Join dialog box opens.
Select the Join with Transition option. The Maximum Angle field enables.
Define the Tolerance and Maximum Angle values.
Note: The values added in the Tolerance and Maximum Angle fields will be saved for future sessions.
Select the first duct and then the second duct.
Transition gets placed joining the selected sides of both the ducts.

Note: If the transition is not placed correctly, update the value of the Maximum Angle.

Example,

If you set the Tolerance 300 mm and Maximum Angle as 30, it will prompt you to select first duct then select second duct. Once you select the two ducts the transition will get inserted to join the two ducts. Here if you observe the transition is not placed correctly, as shown below.

In such case the value of Maximum Angle needs to be updated to fix this issue e.g change the Maximum Angle value to 40.0

After changing the value now, you can observe the correct transition is inserted. So, whenever you observe such issue you will need to increase / decrease the Maximum Angle property input.

Scenario 1:

When we have both the ducts of the same size, and shape and are colinear,

When you use the Join tool for these ducts, it will not insert the transition, but it will merge the two into one duct.

If both ducts are of the same size, and shape and they are not colinear, then the transition will get inserted.