VAT from freelancer to client

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Hi,

I've seen this is a recurring topic, but I'm still very confused about it. Why do UpWork invoices DO mention "VAT reverse charged" once a client based in another EU country is registered for VAT, while they keep silent on the whole VAT subject when invoicing clients who are not registered for VAT.

I am also wondering: if you're a freelancer using UpWork and you're liable to pay VAT in your country of establishment, do you add the VAT percentage to your rate in order not to lose out on another 21% (in my case)?

To be clear: I'm NOT talking about the invoices sent from UpWork to me for service fees, I do understand that part. It's about the invoices they produce from freelancer to client. I am VAT registered myself. 

I was wondering if anyone has any thoughts on this.