Help with LOD expression

Hi,

I'm confused why this LOD expression in this practice used 'MAX', why not 'SUM' or any other AGG functions?

Here is the question and detail solution provided:

Q:

What % of Customers ordering items in 2011 also ordered items in 2012? (use the

customer ID to identify the customer)

A. 49.289%

B. 50.711%

C. 59.71%

D. 43.69%

E. None of the above 

S:

Use a LOD expression to determine whether the customer ordered in 2012:

Filter on 2011 orders:

(this step is where I feel confused, I know this step is trying to figure out who ordered in 2012, but I don't know why the 'MAX' used here)

Filter on 2011 orders: 

 

Please help me with this problem if u have time.

Thank you very much!