Do I need to assign the parent interface when using VLAN?

[testcb00]

Hi everyone,

I have used VLAN for some days and I am learning the basic knowledge about Networking.

In my searching about VLAN, I found that I should not mix untagged and tagged VLAN. Then I started to change the untagged Interface to a tagged Interface.

Currently my Network is configured liked this:

 
And I have changed it to:

However, I found that if I changed the Network Interface from Parent Interface mlxen1 to tagged VLAN mlxen1.50. I can still use the Network without assign the parent Interface mlxen1. Do I need to assign the parent Interface mxlan1, or just leave it unassigned?

Besides, I would like to know why I can mix untag and tagged VLAN in my previous setting.

Thank you.