When asked to comment on the reason that T4A slips are not to include the GST/HST paid to the recipient for services rendered, whereas for T5018 slip purposes, payments made to a subcontractor include GST/HST, CRA stated:
On the one hand, the amount to be reported on the T4A slip is generally an amount prescribed by subsection 153(1) to be included in computing the income of the person who received the amount. … The GST, HST, QST or provincial tax portion, if any, of the amount paid should not be reported on the T4A slip since it is not an amount under subsection 153(1) and is not required to be included in computing the recipient's income under the Act.
On the other hand, the amount to be reported on the T5018 Information Return - Statement of Contract Payments refers to amounts paid or credited for goods delivered or services provided in the course of a construction activity. Thus, the person is required to record payments, including all taxes (GST, HST, QST, or provincial tax, if applicable), made to subcontractors who provide construction services and report them to the CRA. The T5018 Information Return - Statement of Contract Payments is mandatory for any payment for goods delivered or services provided where the total payment is $500 or more in the year. However, no information return is required where the amount is paid or credited in full on account of the sale or lease of goods.
Finally, no information return is required to be filed under ITR subsection 200(1) (T4A slips and the corresponding T4A Summary) in respect of a particular payment made by a person if an information return (T5018 - Statement of Contract Payments) has been filed under ITR section 238 for that particular payment.