You have the RECEIVED status definition, and then there is PRE-PROCESSING... this is where the order is taken from RECEIVED and "staged" for processing. In other words it's added to a QUEUE. In this stage some changes to files can still be made. There are a few differences in the products and what happens in each stage, so these descriptions are a little vague to cover all. In the PRE-PROCESSING stage we are confirming that we have everything needed (in detail) to process the order. We are also checking details to confirm that there are no issues - missing files, wrong files uploaded, hole sizes present, etc.
ENGINEERING is where the files have actually been mapped to layers, everything is present and the board is waiting to be laid up onto a panel for production. There are a few more steps (details) here for PCB Full Feature products than there are for Quick Turn PCB products. At any time during these steps, the order can be placed in a HOLD status; either at your request or other issues such as files, possible production issues, order form clarification, etc. It's basically a 'single line' status movement rather than a tree like structure. I hope this answers your questions - do let me know if you would like more clarification or detail.
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Thank you! Terri Miller