ThisWorkbook is the workbook the code lives in. ActiveWorkbook is whatever is in front, which changes when the user clicks. Code that means "my file" should say ThisWorkbook.

Code

ThisWorkbook.Worksheets("Data").Range("A1").Value = 1

• Explicit

  ThisWorkbook.Worksheets("Data").Range("A1").Value = 1 → Writes to this file regardless of what is active

  Safe.

• Ambiguous

  ActiveWorkbook.Save → Saves whatever is in front, maybe not your file

  Risky.

• Another workbook

  Set wb = Workbooks.Open("C:\data\input.xlsx") wb.Worksheets(1).Range("A1").Copy → A variable for another workbook

  Then both are explicit.

• Prefer ThisWorkbook and explicit sheet names over Active anything.
• Assign opened workbooks to variables and stop using Active anything.
• ThisWorkbook.Path gives the folder the file is in.
• In an add-in, ThisWorkbook is the add-in, not the user's file.

• ActiveWorkbook after Workbooks.Open is the opened file, which is fine until another opens.
• Unqualified Worksheets("Data") means the active workbook.