It's the general (analog) supply voltage, apart from the logic control voltage. It may be uni- or bipolar. It limits the analog output voltage of the DAC.
DAC chips often have separate Vdd pins and also an analog (audio...) input or reference pin. Breakout boards are very limited in the number of pins, perhaps for easy application without specific knowledge. Missing signals don't need a description and simplify advertising.
@DrDiettrich
Please show which module has this connection
E.g. MCP4922.
Please learn more from e.g. this source.
I can't find that module. Can you provide a link?
Within this topic you could not find very much yourself. I'll stop searching the WEB for you now. A last link.
@DrDiettrich
That is an IC not a module. I think you gave the wrong link.
Since post#7 we have been talking about DAC modules not single ICs
I can not find a DAC module with an opamp, do you?
No.
@jim-p @DrDiettrich This is the one i will be using , I think it has opamp: MAX517 https://www.analog.com/media/en/technical-documentation/data-sheets/MAX517-MAX519.pdf?ADICID=SYND_WW_P682800_PF-spglobal
I don't see an Arduino library for that part.
Do you understand how to communicate with it using I2C
The data sheet contains information about MAX517, 518 and 519 chips. Did you fully understand the data sheet? Are you sure that the 517 is the right one for you?
I will use I2C multiplexer, TCA9548A, which i can connect all the DACs through it. regarding Arduino library I'm not sure if i understand what you mean?
Yes I think so, as the info for MAX517 in data sheet regarding the voltages for input and output is the same as what im looking for and it also I2C compatible
If you understand how to talk to the IC then you won't need a library that does it for you.
So you did not understand the difference between the chips 
can you explain in which term you mean? i decided to use I2C multiplexer as its the only way to handle multiple devices with the same address.
All those chips expose a different number of address configuration pins.
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