https://developer.matomo.org/api-reference/reporting-api documents 'period' and 'date' API argument :

• forperiod=range it states an example: « to request a report for the range Jan 1st to Feb 15th you would writeperiod=range&date=2011-01-01,2011-02-15 »
• date documentation includes this :

Note: if you set 'period=range' to request data for a custom date range, the API will return the sum of data for the specified date range. When 'period=range', the following keywords are supported for the parameter 'date':- lastX- previousX- YYYY-MM-DD,YYYY-MM-DD, both dates can be replaced with magic keywords (the first one does not support today and yesterday). eg. YYYY-MM-DD,today, lastMonth,YYYY-MM-DD or lastYear,lastWeek

But whendate islast10, is it 10 days ? weeks ? months ? years ?

Documentation should mention what parameter holds 'day', 'week', 'month' or 'year' whenperiod isrange.

It could bedate=last10 is not possible whenperiod=range, and in such a case documentation should be fixed also.