Purchase Order rate field

Respected Experts, please guide me what is the Purchase Order rate field.

[#Field:VCH Purcorder]
Add:Option:Rakesh@IsPurchase

[!Field:Rakesh]
Validate$$IsEmpty:$$Value or $$Value=$Purchaseorderrate:StockItem:$StockItemName)

Sir, with the help of above code I want to block rate as rate should be equal to Purchase Order.

Kindly guide me where is error.