This property is primarily a "Read" property allowing you to retrieve the number of bytes in the transmit buffer.
The only value that can be 'written' to this property is 0 (zero) which can be used to delete all data from the transmit buffer.
Note for c# developers. In VB6 using MSComm32 this property is a short integer. VB.Net developers don't usually need to worry about things like that but C#, a stronger typed language, may need a cast to/from (short) when using .Net int values.
The behaviour with MSComm32 is the same so any existing code using MScomm32 should not need rewriting.