Explanation:

Comprehensive Detailed Explanation
Step 1: Reviewing the provided DAX expression
SWITCH(
SELECTEDVALUE(Metric[Metric]),
"# of Customers", "#",
"Gross Margin %", "0.00%",
"#,###"
)
This is applied as a dynamic format string.
It changes the display formatting of the Value measure depending on the selected metric.
Step 2: Identify the feature
The screenshot shows Format = Dynamic in the Power BI Modeling tab.
This confirms that a dynamic format string has been applied.
So, for the first blank # dynamic format string.
Step 3: Identify the output formats
If the metric = # of Customers # format is "#" # whole numbers.
If the metric = Gross Margin % # format is "0.00%" # percentages.
Fallback # "#,###" # also whole numbers.
Thus, the measure supports both whole numbers and percentages.
So, for the second blank # percentages or whole numbers.
References
Dynamic format strings in Power BI
SWITCH function in DAX