Placeholder vs Watermark

[mrj001]

Controls such as NumericUpDown, AutoCompleteBox, CalendarDatePicker and TextBox implement a Watermark property.

However, the ComboBox implements a PlaceholderText property instead.

The TextBox.Watermark property documentation offers no explanatory text. There is no documentation on the ComboBox.PlaceholderText property.

It appears to me that this is inconsistent naming for the same concept. However, Issue#1282 contains the following relating to TextBox:

Need to add Placeholder related properties (instead of/in addition to Watermark)

The phrase "in addition to" implies that there is a difference between these two.

What is the difference between a Watermark and a PlaceholderText? Or should the ComboBox be updated for consistency?