Basically stuff that doesn't have enough of a root system yet. Your response really helps me though. Thank you.Most plants have roots that go under the substrate or attach to objects to hold them in place. They shouldn't need anchors.
Mosses and the like can be tied with string or fishing line, sandwiched under netting or mesh, or affixed with gel superglue.
Propagated stem plants that haven't yet developed roots can be weighted with plant anchors until they grow.
What are you looking to plant?