Target HR Zone Upper and lower bounds - My e-training

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Hello,
I'm new to My e-training and noticed that in the profile/settings page are two Target HR Zone figures - upper and lower bounds - which i have no idea what to set to as i do not understand the meaning. I can see that in the same setting page is also possible to specify 5 HR zones (defined by Elite as Easiest to Hardest) and I'm more familiar with those.
Any idea of the meaning for upper and lower bound zones and how they are eventually used by the SW?
Thanks