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In eFX spot markets “last look” is the name given to a window of time where the market-maker has the option to reject the trade. It is a controversial topic and one highlighted within the FX code of conduct.
There are pros and cons to the usage of last look:

Advantages

• Allows liquidity providers to display greater liquidity on each venue.
• Allows generally tighter bid/ask spreads on each venue.
• Protects markets from liquidity arbitrageurs who may cause wider spreads for all market paricipants.
• Allows top of book prices to be more stable (lower update frequency).

Disadvantages

• Liquidity takers have no certainty of fill.
• Liquidity takers need to return to market after a period of time where they are exposed to market movements.
• Trades are more likely to be rejected in volatile markets.
• Liquidity takers need clarity over why a trade has been rejected and assurances that the information about the trade has not been misused.

Academic Paper 

In this paper, Jamie Walton with his co-authors quantify the cost of the last look option and suggest the reduction in spreads that may result as a result of last look liquidity.

• https://papers.ssrn.com/sol3/papers2.cfm?abstract_id=2630662