How do I edit item lines on invoices, credits, quotations and orders?

Depending on the layout type you're in, you can then see the following information:

Units - picked up from the product record. Can be overtyped if required.
Description - picked up from product record although can be overwritten if required - maximum of 60 characters.
Comment 1/2 - maximum of 60 characters each. Cannot be preset.
Order reference - for transaction email use only.
Order Line Reference - shows items position on invoice, for transaction email use only.

Nominal Code - picked up from the product record. However if a nominal code is entered in the Footer Details tab > Global section, then the item line N/C will be overridden when the invoice is updated to the ledgers.
Tax Code - Can be picked up from the a number of areas, however the order of priority is:
1. Delivery Address Tax Code - this does not depend on the override being set in the customer record.
2. Customer Record > Defaults Tab > Default Tax Code - only used if Use Default Tax Code for Sales checkbox is ticked.
3. Product Record.
  
  NOTE: When updating, if a nominal code and tax code have been entered into the global posting details, when the invoice is updated to the ledgers it will be updated with the global tax code. The tax code will only be overridden in this way if a nominal code is entered in global, if it hasn't then the update ignores the global posting tax code. Even if the global tax code is used it does not recalculate the amount of VAT, this is still taken from the invoice item lines.

Units - picked up from the product record. Can be overtyped if required.
Description - picked up from product record although can be overwritten if required - Max 60 characters.
Comment 1/2 - Max 60 characters each. Cannot be preset.
Order reference - for tmail use only.
Order Line Reference - shows items position on order, for transaction email use only.

Nominal Code - picked up from the product record. However if a nominal code is entered in the Footer Details tab > Global section, then the item line nominal code will be overridden when the invoice, created via SOP Despatch, is updated to the ledgers.
Tax Code - Can be picked up from the a number of areas, however the order of priority is:
1. Delivery Address Tax Code - this does not depend on the override being set in the customer record.
2. Customer Record > Defaults Tab > Default Tax Code - only used if Use Default Tax Code for Sales checkbox is ticked.
3. Product Record.
  
  NOTE: When updating an invoice, created via SOP Despatch, if a nominal code and tax code have been entered into the global posting details, when the invoice is updated to the ledgers it will be updated with the global tax code. The tax code will only be overridden in this way if a nominal code is entered in global, if it hasn't then the update ignores the global posting tax code. Even if the global tax code is used it does not recalculate the amount of VAT, this is still taken from the SOP item lines.

Units - picked up from the product record. Can be overtyped if required.
Description - picked up from product record although can be overwritten if required - maximum of 60 characters.
Comment 1/2 - maximum of 60 characters each. Cannot be preset.

Nominal Code - picked up from the product record. However this is overridden by the Supplier Record Default N/C if the Use Default Nominal Code for Purchases checkbox in the Supplier Record is ticked.
Tax Code - Can be picked up from the a number of areas, however the order of priority is:
1. Supplier Record > Defaults Tab > Default Tax Code - only used if Use Default Tax Code for Purchases checkbox is ticked.
2. Product Record.