HI all.
I'm wondering how can I create my own data to use them while creating my application? or should I create them outside, then import them to Appian?
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Are you referring to Custom Data types? Creating via the Appian GUI or instead via an XSD file and importing? In which case, both work fine - the GUI is simpler for design but XSD is used to add more advanced configurations when necessary.
You can use a database, expression rules, and constants for storing your real/dummy data.