endpoint_addr is the address of the Vertex endpoint contracts. Deposits are sent to the endpoint address; this to used sign every request except PlaceOrderand PlaceIsolatedOrder
book_addrs contain the address of each product’s orderbook on Vertex. book_addr[i] is the address of the orderbook for product id i. The quote product (i == 0) does not have have an orderbook, so book_addr[0] is the null address; this is used to sign PlaceOrder and PlaceIsolatedOrder requests for a provided product_id.