Hello
ETO means Engineer-to-order. Basically, in this process the production is controlled by a project.
This is generally used when the product is very complex and it is developed specifically for the customer.
This subject has been already discussed on the following threads:
make-to-order vs. engineer-to-order
Engineer to order process flow
You will also find more information on the links below:
Sales Order Processing with Engineer-to-Order Production - Order BOMs (PP-BD-BOM) - SAP Library
Engineer-to-Order (ETO) of a Turbine - SAP Engineering & Construction - SAP Library
232 – Engineer–to–Order (ETO) - Quotation Processing
BR
Caetano