The LOT_SIZE.maxQty is the maximum allowed “order quantity” (= base currency): in this case: I can buy at most 9.000 ETH per order.

What I need to know is the maximum allowed order value, i.e. how much money (= counter currency) is allowed to spend at most per order. As I understand, PRICE_FILTER.maxQty is only the maximum allowed money to spend for one share and not for the whole order volume, or?

max. 9.000 ETH per order max. ? USDT per order (the ? is what I need to know!),

ETH is the base asset. LOT_SIZE is measured by the amount of ETH, base asset.

USDT is the quote asset. For example when you place a LIMIT order with quoteOrderQty specified, the system will convert USDT to the corresponding quantity in BTC and check the LOT_SIZE. If you placed a MARKET order it would check the MARKET_LOT_SIZE

LOT_SIZE is the maximum allowed order in the base currency, which is 9000 ETH. MARKET_LOT_SIZE is the maximum allowed market order in the base currency, currently 4635.03365625 ETH.

Essentially you would have to multiply the LOT_SIZE or MARKET_LOT_SIZE by the current ETH price which wouldn’t be an exact science.