A "partial until filled" limit order is an instruction to buy or sell an asset at a specified price. The order will continue to be executed until the full quantity is filled. It is also possible that the full amount of the order will not be completely filled based on the order price supply and demand.
For example, let's say you want to buy 100 tokens of ABC at a limit price of $50 per token, and you place a limit order. If there are only 50 tokens available at or below the limit price of $50 per token, INX will execute that portion of the order and fill 50 tokens, while the remaining 50 tokens will remain as an open order until the token price drops to your specified limit price. It is also possible that that price does not drop in which case you can then cancel the remaining part of the order in the above example.
In this way, "partial until filled" orders can allow investors to make purchases or sales in a more flexible manner, rather than having to fill the entire order at once, especially in a market that is somewhat illiquid and volatile.