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Say I have a currency pair GBP/USD at rate 1.45, buying 1 GBP costs 1.45 USD and selling 1 GBP gives me 1.45 USD.
Now I put an order of 100,000 on this pair on the buy side. Does this usually mean (if there even is a fixed convention for this) I buy 100,000 GBP, which means it costs me 145,000 USD, or does it mean I pay 100,000 USD and get 68,965 GBP?
The convention is, buying 100,000 GBPUSD means that you want 100,000GBP. Order size is the base currency amount. It will cost you 145,000 USD if the offer rate is 1.45.
If you want to sell a particular USD amount and let the market rate determine the GBP amount that you will get, it is called "non-base trading" - order size is not the size of base currency. You will set the trading product to USD and quantity to 100,000. Field names may vary between platforms. Or just say "sell 100k USD, buying sterling" in phone order.
Neither. Your order would be rejected without an "amount currency" to specify the size of the quote (or deal) otherwise the dealer can't properly assign a spread to you. Spreads are decided according to asset / inventory in this case either GBP or USD and the dealer needs to know which one.
What you're touching on is market convention about which way around dealers like to deal with currency instruments. There's an established order, 1 underlying reason for that is which way round is it easier to do the conversion maths... but again whether you're priced in GBPUSD or USDGBP you need to give your order a size either in USD or GBP.
