I hope im right.,.. im pretty sure i am anyway, all it means is its readily absorped by the water and the plants.. i think the term is also chelated, there are types or Iron (Fe) that cannot be dissolved as easily as chelated iron therefor leaving deposits etc which are useless.
i also think the process of hydration involves taking the solid to a liquid.. then back to a solid again. but containing only the watersoluble solids that will assure 100% disolution.. and leaving the unsoluble chemicals for other purposes. i might be wrong but someone on here will sure answer you.
but you have the right idea.