A client recently asked to have a retention for mail items but to leave calendar and tasks untouched. When in the ECP the option to create a specific retention tag for calendar items or tasks is missing. You must do this through Powershell.
Below is the command I used for both Calendar items and Tasks
New-RetentionPolicyTag “Name of Retention Policy Tag” -Type Calendar -RetentionEnabled $false -RetentionAction DeleteAllowRecovery
Do the same for Tasks replacing Calendar for Tasks in the Type field. Once run you can then add these Retention Policy Tags to your Retention Policy.