How do I know which parameters are required and which ones are optional?

For items array, all the parameters are required except for the following:

<Parameters that are low-priority and optional for items array>
- externalVendorSku: item code used by the seller/vendor 
- barcode 
- emptyBarcode: no barcode available
- emptyBarcodeReason: reason why no barcode is available
- modelNo: model number
- extraProperties: additional option info about the seller's item 
- certifications: any certification about the item
- searchTags: search words