If you're providing services to your clients that shouldn't have Sales Tax applied, you'll need to ensure that the associated Work has been setup accordingly.
Under Manage > Work create a new Work for those services, apply the appropriate settings to the Work (for more detail on creating new Work 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 you'll need to edit the placement and add the Work to the Work tab on the placement:
Any time recorded against this Work for hourly/daily placements, or any placement fees charged, will now be taxed accordingly when they are billed.