From the Knowledge Base: The asterisk is actually a visual indicator of a special type of Null value that occurs when there are multiple members that apply to the mark.
Meaning, you have multiple data values and Tableau doesn't know which to display.
Please see the following to help you along: Troubleshoot Data Blending
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sounds like you blended the data - the * indicate there are multiple date records instead of a unique value
don't know your data so it is hard to tell what to do - could be restructuring the data or using connecting with a join would work better
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I guess the granularity of each data source is different that's the reason when you're using any data field from secondary data sources its returning "*". You need to aggregate your data sources and make sure level of detail of each source should be same then you'll get the proper result. Instead of blending you may try with joining data sources and check.
Typically, when there are multiple matching value in the secondary data source for each mark in the primary data source, asterisks in the sheet are displayed.
You can avoid asterisks in the sheet by making sure that there is only one matching value in the secondary data source for each mark in the primary data source.
The article below document when asterisks are displayed in the sheet and how to resolve asterisks in the sheet
Hope this help.