PricingCurrencyCode vs. LineExtensionAmount.currencyID

[ericdes <eric@vcardprocessor.com>]

(This question could also apply to other document level specifications 
of currencies vs. individual monetary amounts where the currency codes 
are specified.)

Hello
I took this example from the document Invoice: each amount specifies a 
currency ID, which raises a few questions: How does the document level 
PricingCurrencyCode relates to those amounts? How do you handle possible 
discrepencies? Why specifying a currency ID at each monetary amount line 
if they're all the same as PricingCurrencyCode?

I'm getting confused at these document level indications I hadn't 
noticed before...

Any help would be appreciated,
Thank you,
Eric Desgranges.