Meaning of underscore( _ ) in a!pickerFieldCustom Component

I have a question while using the a!pickerFieldCustom Component example in Document.

a!localVariables(
  local!pickedState,
  local!stateLabels: { "Alabama", "Alaska", "Arizona", "Arkansas", "California", "Colorado", "Connecticut", "Delaware", "Florida", "Georgia", "Hawaii", "Idaho", "Illinois", "Indiana", "Iowa", "Kansas", "Kentucky", "Louisiana", "Maine", "Maryland", "Massachusetts", "Michigan", "Minnesota", "Mississippi", "Missouri", "Montana", "Nebraska", "Nevada", "New Hampshire", "New Jersey", "New Mexico", "New York", "North Carolina", "North Dakota", "Ohio", "Oklahoma", "Oregon", "Pennsylvania", "Rhode Island", "South Carolina", "South Dakota", "Tennessee", "Texas", "Utah", "Vermont", "Virginia", "Washington", "West Virginia", "Wisconsin", "Wyoming" },
  local!stateAbbreviations: { "AL", "AK", "AZ", "AR", "CA", "CO", "CT", "DE", "FL", "GA", "HI", "ID", "IL", "IN", "IA", "KS", "KY", "LA", "ME", "MD", "MA", "MI", "MN", "MS", "MO", "MT", "NE", "NV", "NH", "NJ", "NM", "NY", "NC", "ND", "OH", "OK", "OR", "PA", "RI", "SC", "SD", "TN", "TX", "UT", "VT", "VA", "WA", "WV", "WI", "WY" },
  a!pickerFieldCustom(
    label: "State of Residence",
    instructions: "Value saved: " & local!pickedState,
    placeholder: "Type to select the employee's state of residence",
    maxSelections: 1,
    suggestFunction: rule!ucArrayPickerFilter(
      filter:_ , 
      labels: local!stateLabels, 
      identifiers: local!stateAbbreviations
    ),
    selectedLabels: a!forEach(
      items: local!pickedState,
      expression: index(local!stateLabels, wherecontains(fv!item, local!stateAbbreviations))
    ),
    value: local!pickedState,
    saveInto: local!pickedState
  )
)

I wonder what the meaning of underscore ( _ ) in line 11 in this code.

Below is a link to the page I referenced.

https://docs.appian.com/suite/help/21.3/recipe-configure-an-array-picker.html

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