If you're providing services to your clients that shouldn't have Sales Tax applied, you'll need to ensure that any time for those services is recorded against a work type that's been setup accordingly.
Under Manage > Work create a new work type for those services, apply the appropriate settings to the Work type (for more detail on creating new work types see this article), and ensure the Tax Rate is set to the appropriate setting.
If your account is Xero integrated the Tax Rate will default based on the Account Code you've selected - it's up to you whether you'd like to use a standard Account Code and override the Tax Rate, or if you'd prefer to setup a separate Account Code in Xero which has the appropriate tax status.
Once you've created the work type you'll need to edit the placement and add the work type to the Work tab on the placement:
The Resource can now record their time for the placement against this work type and the invoice lines will be taxed accordingly.