Sets an attribute onto its host element whose value will be the value of a Screen data variable.
The primary attribute expression of this command is expected to be a String of a certain format defining both the attribute to set and its value. The format is as follows:
attributeName=attributeValue
The 'attributeName' is simply the name of the attribute you wish to set, while 'attributeValue' is the expression whose result you'd like to see as the attribute's value.