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.