First, separate support from decoration
A tree stand holds the tree upright. A collar disguises the stand with a rigid surround. A skirt is a flexible fabric layer placed around the bottom. A collar or skirt usually does not replace the supporting stand: buying one because it looks like a base can leave you missing the part that matters.
For a real cut tree
Choose a stand specified for the height and trunk diameter of your actual tree, with a water reservoir. Keep its controls and watering opening accessible after decorating. A potted, rooted tree is a different setup: it needs a suitable container and its own care instructions, rather than being clamped as a cut trunk.
For an artificial tree
Use the supplied stand or a replacement explicitly compatible with the tree. Height alone does not establish compatibility: the pole diameter, retention mechanism and load rating matter. A cover should fit around the fully opened stand without pushing its feet inward.
Choose your finishing layer
A collar gives a neat, structured edge and hides cables, provided they remain accessible. A skirt is easier to fold away and can be wrapped around an already decorated tree. If you combine both, check that fabric cannot catch in the stand mechanism. Measure before ordering; the three items solve three different jobs.