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When I used this code CONCAT('<b>','Hello World','</b>') in the card Textbox (Table) or in Single Value card in Gauges, its giving me text as is "<b>Hello World</b>" while my expectation is that it should bold the "Hello World".
Is Summary Number a different type of card if so where would I find it?
Are you saving the card after you've applied the summary number? You will continue to see the HTML code until you save the card to which it should display as expected.