Tax instalments are payments you make throughout the year to cover the taxes you would otherwise pay in one lump sum after the year ends. You pay them while you are earning the income, much as an employer deducts tax directly from each pay period. An Instalment Reminder is the letter the Canada Revenue Agency (CRA) sends setting out the instalments it suggests you pay.
What this document is
A reminder with suggested amounts, not a bill for tax already assessed. It tells you that the CRA expects instalments from you and what it calculates those instalments to be.
You may have to pay instalments for next year's taxes if your net tax owing is over a set amount in the current year and in either of the two years before it. The CRA's page sets out that amount and the factors considered.
When the payments are due
Instalment payments are due on four fixed dates: the fifteenth of March, June, September, and December. Farmers and fishers are the exception — they have one due date, at the end of December.
The CRA's page also sets out options for calculating the amount, which can reduce or eliminate what you pay, and explains how instalment interest and penalty are charged when payments are late or short.
What it supports
Instalments you actually paid during the year are claimed on your return, in the payments section. The reminder is the CRA's side of the arrangement; your payment records are your side, and it is the payments — not the reminder — that the claim is built from.
What to keep
Keep the reminder with the year's payments. On its own it shows what was suggested; paired with your confirmations of payment, it shows what was actually paid and when.
Keep both for every year you pay instalments, because the amounts you paid feed the payments section of that year's return, and because the CRA's page has a section on verifying payments and on what to do if you overpaid.
Dossier keeps the reminder and the year's payment confirmations together, so the total that goes on the return is there when you file.
Official CRA sources
Source: Canada Revenue Agency (canada.ca). Not tax advice.