velcro on the mattebox

They're filter tags. It let's the DP know what's in the mattebox at any given time. Most rental houses will have these tags on the filter pouches.
 
Especially when using a lightmeter, it's very important to always compensate for any mounted filters in the reading. If you were to forget you had a filter in the mattebox you might underexpose, or stripping it without telling it will be blown. Having tags will help everyone know what the rating for the current setup is.
 
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