Hello,
The field will be fill with the category that the vendor was browsing (in the market product listing), so it's possible to have something else.
We recommend you use the setting "Change vendor root category" to avoid vendor creating products in your store main product category.
You can also use product templates in order to provide default values for the product category field.
Otherwise it's still possible to use a view override in order to empty the product category when you're creating a new product (the product_id will be empty).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.