how to use spellFields

When I turn on 'Spell chess' rule in a custom variant and use edit position, I find the spellFields option in the FEN. Does anyone know what is it used for and how to use it (. what to write in the bracket) ?

spellFields is an array that sores spells that are on the board right now, it is possible to manually cast a spell anywhere on the board with this array, but it will always reset when you start a game making it practically useless

Thanks. Then is there any method to create a starting position with spells on the board?

You can make a bot move first and give it some spells, but it will place them randomly

But can bots place spells? When I test in 'analysis', bots just get stuck when they try to place a spell.

They can place spells, but it lags before they place the in analysis

i never use FEN