Hi @Daniel_Shin, I hope that you are well!
Thanks for asking the HubSpot Community!
The article “Manage notifications in Slack” has been updated and I can see that “once a company or deal has been associated with a channel, all new calls, notes, emails, tasks, and meetings on that record, and contacts associated with those company or deal records, will trigger a Slack notification. You can associate multiple records with the same Slack channel”.
Also, you might be interested in this: “7 Slack Tips & Tricks and HubSpot Integration”.
I hope this helps!
Have a lovely day! 
Best,
Bérangère