How it works
The two are stages in one handoff. Marketing decides who counts as an MQL, often with a score: points for visiting the pricing page, for opening emails, for a job title that fits. Sales decides who counts as an SQL, usually after a first call confirms a need, a budget and someone who can say yes.
Some teams add a stage between them, the sales accepted lead, or SAL. Sales has agreed to work it but hasn't qualified it yet.
Both sides have to write the rules down and agree them. Without that, marketing reports plenty of MQLs, sales says they're no good, and each side is right by its own definition.
Worked example, with made-up figures: marketing passes 200 MQLs in a month. Sales accepts 120 and qualifies 30 of those as SQLs, and 6 of the 30 close. MQL to SQL is 15%, SQL to closed is 20%, and each closed deal took about 33 MQLs.
For your calendar, the SQL count is the one that predicts meetings. If MQLs double and SQLs stay at 30, your week doesn't change.
Where it shows up in your week
The handoff is where interested people go cold. WhatsApp workflows send new enquiries a first message from your own number and pass the hot ones to a teammate, so nobody waits for sales to notice them.