Is selaginela supposed to be planted upright to grow tall or just left on the soil to make a carpet ?

I'm a bit confused as to how I am supposed to plant these in a terrarium ? They are called "peacock spikemoss" and described as a groundcover which suggests to me that they should be planted similarly to what is show In the picture. But everytime I see people using it in YouTube videos they seem to just take a piece and plant it upright so it grows up and tall.