How to know if you're adding liquidity? What are Tape A and Tape C?

Tape A = NYSE-listed securities
Tape B = NYSE Arca + Nyse Mkt (formerly AMEX)-listed securities
Tape C = NASDAQ-listed securities

Adding liquidity:
Buy limit order below ask
Sell limit order above bid
 
On your platform you should also see liquidity flag in executions - which show if you add or remove.

That is not typical. Your broker must have added that feature.
 
Adding liquidity: when you place an order that is being booked, and trade later.
Removing liquidity: when you place an order that is filled immediately.

As simple as that.
 
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