encapsulation dot1q <vlan id>- confusion

foreverlearning

I am confused about what this command does.

Does it 
1) Allow that vlan id to pass through 
OR 
2) it encapsulates the original vlan in another vlan?

If it is scenario 1 as explained by my colleague,
how is this command different from switchport mode trunk allowed vlan <vlan id>?
Why is there encapsulation if it is just a vlan being allowed to pass through?

If it is scenario 2,
then is encapsulation dot1q same as qinq terminology? 

Spiegel

They are both essentially the same same thing. Basically that encapsulation command allows you to use 802.1Q to tag an Ethernet frame with a vlan tag. Cisco used to encapsulate with ISL but that is no longer supported on newer Cisco devices, therefore switchport mode trunk does automatic 802.1Q encasulation.