Find the last row in Excel that has data - What are the Range paramaters

Sub ImportNewSSN()
Dim ThisWS As Worksheet
Dim WS As Worksheet
Dim WB As Workbook
Dim WBPath As String, WBFile As String
Dim ThisMaxRow As Long, MaxRow As Long, I As Long
Dim SSNCount As Long
Dim cCell As Range
Dim X
Dim LstRow As Long

Application.EnableEvents = False
Application.DisplayAlerts = False
Application.ScreenUpdating = False

Set ThisWS = ActiveSheet
ThisMaxRow = ThisWS.Range("A:A").End(xlDown).Row + 1
WBPath = ActiveWorkbook.Path

Do
    WBFile = GFileName(WBPath, "Please choose SSN File to Import: ")
    If WBFile = "" Then
        If MsgBox("No file has been selected" & Chr(10) _
            & "[OK]     to continue and select a file." & Chr(10) _
            & "[Cancel] to Exit." & Chr(10) & Chr(10) _
            & "Please make a selection.", vbInformation + vbOKCancel, "Import SSN File") = vbCancel Then
            Application.EnableEvents = True
            Application.DisplayAlerts = True
            Application.ScreenUpdating = True
            Exit Sub
        End If
    End If
Loop Until WBFile <> ""
    
Set WB = Workbooks.Open(WBFile)
Set WS = ActiveSheet
LstRow = WS.Range("A100000").End(xlUp).Row

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