I have heard the sentence
Qual seu trabalho?
Why am I allowed to omit the verb ser here? I alredy know that you are allowed to leave out the definite article, but why the verb?
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TL;DR: The phrase
Qual seu trabalho? is allowed because there is loss of meaning by the verb omission but implies a spoken context.
In a more formal context, the verb has to be added. The article
o also can be omitted because of the casual context, but in some regions may sound odd.
It is permitted under colloquial language. The verb is implied in the phrase. Under formal grammar, the verb would have to be added. Some authors
If it is a narrative text, the colloquial use would be accepted as is.
On a more formal context, like an interview transcript, if the speaker said such phrase it would be redacted as either:
Qual [o seu] trabalho?
Qual o seu trabalho? (sic)? 
The editors would add the verb under brackets or the (sic) indicator, to dispel any suspicion of an error in the transcription.