I created a new Decimal Accuracy with named 'purchase' and digitis = 4 , I update the code to : price_subtotal = fields.Monetary(compute='_compute_amount', string='Subtotal', store=True, digits = 'purchase') , but doesn't work , @Ray Carnes (ray)

I see now you are referencing a MONETARY field. You are better making your own float field as the decimal precision is defined by the `currency_field` attribute. See the documentation at https://www.odoo.com/documentation/14.0/developer/reference/orm.html?highlight=monetary#odoo.fields.Monetary

@Ray Carnes (ray) , can you give me an example please , how to redefine the field price_subtotal in 'purchase.order.line' model

You should NOT redefine the field. Create your own. The value of YOUR field can be taken from the original, and shown in any precision you decide.