Production number is assigned from sequence on creation of the production.
I suspect most companies will want to have continuous numbers without gaps.
Some (my accountant) might also want the numbers sorting to also be in chronological completion (as opposed to creation) order.
The way I use production is directly, that is we do not plan production first on paper, and then introduce the productions in Tryton, we use tryton to plan the order of productions.
This means that sometimes we have to delete productions, or reorder them, leaving gaps in the numbers and making the numbers not chronological.
For example in sale, the number is generated on quotation, so while the sale is in draft mode you can play with it without messing up the numbers.
Of course I can override when the production numbers are generated (on assigned state?) in a custom module for my own use, but I am wondering what the rationale is for generating the number on production creation.
We’ve found that when using stock_supply_production module the productions are deleted and recreated each morning if supply wizard is activated.
This causes the production number to grow to bigger numbers fastly without any sense. So setting the number when the production is set to waiting will also improve such behaviour.
I’m proposing waiting as this is the first state a someone should consider a production to be ready to work on. On previous states (request, draft) the production is temporal and not definitive.
Seams like you are using the production number as if it’s the lot number of the output. Remember you can assign the lot number when finishing the production.