From where are the tax codes in an invoice derived?

I have created many invoices (some are from converted sales orders or quotes), and I have recently noticed during searches and adjustments that the tax code column following my application of a labour charge will sometimes display the code for 'HST included'. Normally, it's displayed with the 'HST to be applied' code, i.e.; the HST is NOT included in the amount for labour displayed.

I have one labour code that I access and insert in the Item No. column, so I can't understand where the inconsistency is coming from. I sometimes have inventory items included in the invoice's details to be charged to the client, but even those normally appear with the 'HST to be added' code, so any shuffling or editing of the invoice before adding labour charges doesn't seem to account for the switch from 'not included' to 'included'. 

I have been correcting this mistake manually, and along with the failure of SA to add a code to every line of labour charges within a single invoice (see my previous posting on this), this has become that much more frustrating.

Suggestions?

Each Customer and Vendor can have a default tax code set for them.  You can do this in the Taxes tab of their Customer/vendor record.  This tax code would automatically appear when you select an item on an invoice for that Customer/Vendor. 

Inventory items and Services also have individual tax settings.  You can make an Item or Service exempt from any tax by opening the Item/Service record, going to the Tax tab and setting the appropriate tax to exempt. 

You should also be aware that, when adjusting an invoice, you can only use a tax code that existed (along with all its settings) at the time that the original invoice was created.  If you subsequently changed the settings of a tax code or you created a new one, these will not be available when adjusting.  In this case you have to completely reverse the invoice and re-post it.

Hope this helps a little. 

Thanks.

I have done these checks through my entire Customer and Inventory ledgers and everything appears to be as it should be. No recent changes have been made to the tax codes. It's all very clean looking. However, I am sure I have missed something, somewhere. Interestingly enough, I have not had a recurrence of this particular error in invoices I have created since my origianl post. Perhaps the problem lies with something I had done at the time, and as yet have not repeated.

Thanks again.