Ok...i went over it earlier in the thread and I am going to do it but not as part of this build. the guttering was not part of the contract as I want a specialist in for it.
Basically, water harvesting is... making use of all water that enters your plot.
You can use waste water from say, showers and washing machines for watering the garden, even flushing toilets, as long as it is thoroughly filtered
you can use rail water for all of the above, plus drinking (through the correct filters), inputting the washing machine, even input to the shower or bath.
If you design the pipework to cater for the future (I did) you can add any or all of this at a later date.
You need a harvesting method, say a gutter system, a storage capacity to suit your needs (tankage) a pump system to circulate the collected water and input it back to your domestic use.
if you want toilet flushing, shower use and drinking water, you can't have a single tank if you are using both sink waste water and rain harvesting.
If you decide to use the sink / shower / washing machine water solely for irrigation (good idea with all those phosphates) life is easier. The rainwater supply can achieve as much of the other options as you want, depending on supply volume of course.
Just my ramblings, not cast on tablets of stone...