I had been confused by this sentence:
Attach the bottom portion (copy 2) of this receipt to your completed application.
until I saw the paragraph as below:
The following list indicates what you should be doing with the different parts of
the IMM 5401 receipt.
Copy 1 (top): Keep for your records.
Copy 2 (middle): Attach to your completed application.
Copy 3 (bottom): The financial institution will keep the bottom part.