For a dev, QGIS 2.99.0-Master is the next QGIS version which will be released.
that why http://doc.qgis.org/api/index.html is so important to make the bridge.
But it is simple as that ?
What will become 2.99 when QGIS 3 will be released ? Is the dev version of QGIS 3.x will be named QGIS 3.99.0-Master ?