In these cases, all it does is encoding a different amount of information right in the name.
In the first example you have both the underlying interface (vmbr0) and the VLAN Tag in its name. As such both options are greyed out and can't be edited.
In the second example you only have the VLAN Tag encoded in the name, the underlying interface (vmbr2) can still be changed.
When using the GUI it simply creates those entries in /etc/network/interfaces for you. It can be quite convenient in contrast to writing the entries yourself.
/etc/network/interfaces can be a little bit more flexible compared to the GUI if you need it.
If you're unsure, I'd always recommend using the GUI for adding bridges and VLAN interfaces.