tibco.env.HEAP_SIZE is a defined environment variable which can be used in other properties. It will not affect anything if it is not used.

For example if you have tibco.env.HEAP_SIZE set as follows:

tibco.env.HEAP_SIZE=256M

You can make use of this variable as: 

java.heap.size.max %HEAP_SIZE%

The value of java.heap.size.max would be 256M, the same as the value of tibco.env.HEAP_SIZE. If you do not use tibco.env.HEAP_SIZE it will not take effect anywhere in the tra file.