UserForm_Initiaize Vs. UserForm_Activate

My question is about the difference between UserForm_Initialize and UserForm_Activate. I don't understand what sets them apart. Or why they are used. The reason I ask is, I have made 5 ComboBoxes. Array values are assigned to each combo box.

However, which particular array values are assigned to ComboBox2 are dependent on what the user chooses in ComboBox1. And which array values are assigned to ComboBox3 are dependent on what the user chooses in ComboBox2, and so on and so forth, all the way up to ComboBox5.

My understanding is that all ComboBox values are assigned/populated in the UserForm_Activate procedure. Is this correct? If so, am I right to believe the best/only approach to creating the ComboBoxes I described is by calling the UserForm_Activate procedure everytime a ComboBox value is chosen by the user?