Price Open is the opening price of the bar.
Price Buy is the opening price of the position.(It is called "Price Buy" because we have to set the long position only.)

The opening price of the position is determined in the Open Slot. Price buy can be anywhere in the bar while price open is always in the beginning of the bar. In those cases that we have in the open slot - price open (all positions will be opened at the beginning of the bar) price buy is the same as price open. In all other cases price buy is not the same as price open.

Price Open and Price Buy can be used in the indicators' logic.

Price Open is the first price of the bar.
For example:
Filter Moving Average
Price Open is higher then the Moving Average value.

Means - the opening price of the bar is higher than the MA value. When the bar opens above the MA we will be allowed to open a long position and when the price open is below the MA we are allowed to open a short position.

----------------

Price Buy is the price of opening the position. It is determined in the Open Slot. Because we have to set the long position logic only it is called "Price Buy". Of course for the short position it is Selling Price. 

Filter Moving Average
Price Buy is higher then the Moving Average value.

Means - When the price determined in the open slot (this is the price that we will open a position on it) is higher than the MA value, we will open (add) Long. If it is lower than the MA value we will open Short.

When Price Open is set in the Open Slot - "Price Buy" and "Price Open" are equal.

We have this example for a logic that actually can never be executed:

Open  Moving Average (MA1)
...
Period 10

Filter  Moving Average (MA2)
Price Buy is higher then the Moving Average value.
Period 10

Means - We will open Long when MA1 > MA2 and Short when MA1 < MA2
But, this is impossible because both MA are equal.

. . .

If i?m wrong some will correct me
but i sure i?m not.