`STARTINDEX is standard in WFS 2.0, but it's an extension for WFS 1.0 implemented in QGIS Server.
STARTINDEX can be used to skip some features in the result set and in combination with MAXFEATURES provides for the ability to use WFS GetFeature to page through results. Note that STARTINDEX=0 means start with the first feature, skipping none.
This feature was developed by 3Liz