What does the volume number indicate? Lots?

Volume is the count of price "ticks" your Dealer sent out during the bar.

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On the M5 chart I sometimes see VOLUME 145? Does this mean 145 lots traded during the line segment?

No.

Is this representative on the entire Forex, or just a small percentage?

Your price feed only.

What is the percentage compared to the overall Forex trading for the line segment?

Not applicable.

I am just trying to understand the relevance of the volume number.

Now you know.

if Volume is the count of price "ticks", then what is tick ? it is the count of marketmaker send out price to his clients ?

or the count of trade times of his clients ?

in other, tick or tick size is one pip's value. so what is tick? and what is tick count ?

Let's find out.