Climatological normal and anomalies


I'm not expert in climate science and I need your help to solve some doubts in the most simple possible way to understand.

1) The CDS Toolbox documentation states the current climate normal period is January 1981 to December 2010. Is it a worldwide convention? What is the reason behind choosing this period of time? 

2) Regarding climatological normal convention (1981-2010), does it not affect or matter that "the data object" period may be outside 1981-2010 range? Can the data object be just a year?

3) In this section, what is the difference between "ct.climate.anomaly" and "ct.climate.relative_anomaly"? How do the outputs look like? and how they can be interpreted?

Any help will be appreciated

Thanks in advance,