How is this different to Command Planning?
The difference is in how prices, production, consumption and income are decided.
In a centrally planned economy, data is collected and prices are set by a central planning board. Whereas, in a Participatory Economy, prices are set through annual democratic planning which all actors in society participate in.
This has many implications, which you can read about in articles on the site and on the comparison table. Annual democratic planning is designed to solve the inherent failures with central planning and further the values of equity, solidarity, diversity, self management, efficiency and environmental stewardship.