Capacity is an Integer property used to indicate the number of bytes allowed when allocating bytes to the internal storage for the buffer. Capacity indicates the maximum byte count permitted (but not necessarily in use) for the internal storage area. 

Reading the value in Capacity returns the number of bytes currently allocated to the internal storage for the buffer. 

Changing the value in Capacity causes dead space already removed from the head of the buffer using Extract to be compacted, and to optionally add the number of bytes in GrowthFactor to the tail of the internal storage area. 

Use Size to determine the number of bytes currently in use in the internal storage area following Write, Extract, or Remove method calls.

TIdBuffer.Size 

Write 

Extract 

Remove