I am going to try with GDAX. In this article, there are several steps
You can fund your GDAX account by sending digital currency from another exchange or wallet provider.
Go to gdax.com/trade
Choose a trading pair with the currency you wish to deposit from the "select product" menu. If you want to deposit ETH, select ETH/USD or ETH/BTC.
Select the Deposit button at the top left.
Select the "BTC address", "BCH address", "ETH address", or "LTC address" tab.
Copy the account address shown. This address will be permanently associated with your GDAX account. Any digital currency sent to this address will be added to your balance.
Open the external wallet you would like to deposit from, and send to the GDAX account address provided.
My question is in step 6, what is the external wallet? Is it a hardware or software? Is it from the third party or from GDAX itself?