SPO2 and HR can be observed simply by placing your finger over the MAX30101 module on board the MAXREFDES100#. R-to-R intervals and ECG are measured by the MAX30003 AFE.
We here at Maxim believe that giving the user the ability to map the electrodes where they think would give them the best results is a competitive advantage to them. We therefore advise users to read up on readily available knowledge of ECG mapping on the internet. For example, Wikipedia has a great article on ECG mapping and is located here: https://en.wikipedia.org/wiki/Electrocardiography
From experience being in the industry, we know that Lead I (LA-RA) is suggested in mobile device applications because of its ease of use. Also, see this knowledgebase article for MAX30003 connections.