4-20mA source and sink

Hi All, I am trying to understand 4/20mA outputs. I have two questions
1) Why do some output cards have +/-20mA capability as opposed to just
4/20mA capability? Why the ability to sink as well as source and what is this capability used for?
2) If the analog output card needs to sink 4/20mA is it required to be able to work with a grounded load.
Thanks
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a) +/-20mA is an older instrument standard that is not very common nowadays but is still widely used, so manufacturers still support it. b) Some field devices are externally powered and some are loop-powered. The loop-powered ones won't work on current sink inputs...

No.
Hope this helps,
Cameron:-)
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