Floats are elements like tables or figures that are placed (and numbered) automatically by TeX.
Floats are ConTeXt's "base class" for everything that uses a \place... command (table, figure, own floats).
If you search options of e.g. \placeexternalfigure you must look after \placefloat in the manual! (same for \setup... etc.)
place is one of: (PLEASE SOMEONE ADD DESCRIPTIONS!)
caption You get no caption with none.