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In the VBA Editor, highlight the UBound term and press the F1 key for a description.

The Scripting.Dictionary doesn't have VBA help. It's a special array that has a very fast method to compare Records. It's used in this case to store the 1st sheet ID's (Refs) along with their row numbers (i). Then the 2nd sheet's ID's are compared to the Dictionary to find matches. Matched rows (i and k) are then compared cell by cell (j = columns).

Sub Compare_Records()
    
    ' Compares records (rows) from two worksheets based on matched record IDs
    
    ' For each macthed record pair, their cells (columns) are compared.
    ' Differentiated cells are highlightd in yellow on both sheets.
    ' It assumes the two sheets have identical data column configurations
    ' Unmatched record IDs on both sheets have their entire row highlighted.
    
    ' Adjust the worksheet references (w1 and w2)
    ' and the column number (colID) that contains the record IDs to suit.
    
    Dim w1 As Worksheet, w2 As Worksheet
    Dim v1 As Variant, v2 As Variant, v As Variant
    Dim i As Long, j As Long, k As Long, colID As Long
    
    Set w1 = Sheets("Sheet1")   'Master records sheet
    Set w2 = Sheets("Sheet2")   'Comparison records sheet
    
    v1 = w1.Range("A1").CurrentRegion.Value 'Master records data
    v2 = w2.Range("A1").CurrentRegion.Value 'Comparison records data
    
    colID = 1   'Column number that contains the record IDs to match between the two sheets; column A = 1
    
    With CreateObject("Scripting.Dictionary")
        .CompareMode = 1
        'Store the Master sheet record IDs with row numbers
        For i = 2 To UBound(v1, 1)
            .Item(v1(i, colID)) = i
        Next i
        
        Application.ScreenUpdating = False
        
        For i = 2 To UBound(v2, 1)          'Loop through Comparison sheet IDs
            If .Exists(v2(i, colID)) Then   'Find a record ID match if any
                k = .Item(v2(i, colID))     'Master sheet row number of matched ID
                .Item(v2(i, colID)) = 0     'Tag record ID as matched
                For j = LBound(v2, 2) To UBound(v2, 2)  'loop through matched record columns
                    'Highlight differentiated cells on both sheets
                    If v1(k, j) <> v2(i, j) Then
                        w1.Cells(k, j).Interior.Color = vbYellow
                        w2.Cells(i, j).Interior.Color = vbYellow
                    End If
                Next j
            Else
                'Highlight rows on Comparison sheet that had no record ID match
                w2.Rows(i).Resize(, UBound(v2, 2)).Interior.Color = vbYellow 'No ID match
            End If
        Next i
        
        'Highlight rows on Master sheet that had no record ID match
        For Each v In .Items
            If v > 0 Then
                w1.Rows(v).Resize(, UBound(v1, 2)).Interior.Color = vbYellow 'No ID match
            End If
        Next
        
        Application.ScreenUpdating = True
        
    End With
    
End Sub