Some places call it 'optional regime' but I'm always a bit wary of using 'regime'.

https://www2.deloitte.com/content/dam/Deloitte/global/Docume...

There is also an optional simplified regime on income from lucrative activities with rates ranging from 5% and 6% in 2013 and from 5-7% starting 2014.

http://www.knowyourcountry.com/guatemala3.html

Simplified optional system: 7% on gross income

http://taxsummaries.pwc.com/uk/taxsummaries/wwts.nsf/ID/Corp...


--------------------------------------------------
Note added at 26 mins (2016-11-16 20:48:50 GMT)
--------------------------------------------------

Guatemala (tax rate = 31% or 5%)
The Guatemalan corporate income tax system is based on the territoriality principle; all Guatemalan-source income is taxed. It is the choice of the taxpayer to be taxed at a 5 percent rate on gross income (the general system) or 31 percent rate on taxable income (the optional system). Under the optional system, corporate income tax is paid annually but quarterly advance payments are required. Under the general system, corporate income tax is paid on a monthly basis. 

https://www.kpmg.com/Global/en/IssuesAndInsights/ArticlesPub...