It's not so much what a soilless mix IS, but what it ISN'T. It ISN'T a mixture of mineral soil (garden soil/topsoil) and other amendments. Soilless mixes simply contain no mineral soils, yet they can contain other mineral ingredients like Turface, granite, perlite, pumice ........ so go figure. ;o)

I would consider MG soils as soilless, but I don't think that particular designation necessarily defines them as a good choice for container culture.

Seed starting media should be sterile and very well aerated, so most 'from-the-bag' media that are not designated as sterile are probably not the best choice for seed starting.

What you need to know about it is that it is very water-retentive and doesn't retain its structure well, so you'll need to be careful about how you water when using it. This is particularly true when the planting is young and nut well-established.

Al