What does ‘save macros & build nutrition plan’ do

From your 'My Hub' page (please see below)

On the left-hand side of your account, you will see the side menu where you can select 'Nutrition' and then 'generate macros'.

You can select 'Generate Macros' on the nutrition drop-down menu on the left-hand side of your page.

Here you can click the 3 horizontal grey dots to the right of each client name and click 'Generate Macros'

You can then make sure all details are correct for that client or add them in yourself along with the formula you would like the system to use.

Here you will also need to add your client's body type and training frequency.

Then click on 'Generate Macros'

You will then be presented with the results 

If you click 'Build Nutrition Plan From Macros' this will allow you to add food items to the plan to match the daily macro targets.

Just remember to hit the 'Save Plan' button.