What is a custom task in SFDC?
If you are fully integrated with Salesforce, you can mix custom Salesforce tasks into your campaign to create any touch type you'd like.
If you choose Custom task in SFDC for a Touch, Sendbloom will create tasks for you to complete the action you describe in the task subject (i.e. tweet at your prospect, research their competitors and write personal email, etc.) for every prospect that has not replied to the campaign.
What you'll see in Salesforce
Custom tasks have a reminder date of the date of the Touch and have a subject of whatever you designated in the "subject" of your custom task. You can customize any field of call tasks (i.e. task type) in your Salesforce object field rules in Settings.