Expressions are central to using Strategy Manger and how you make use of attributes; you can use an attribute by inserting it as the expression, or part of the expression, of a process modifier. Expressions also appear in data nodes and decision nodes.
Here are some expressions setting modifiers of a Hole cycle process:
Modifiers can be set by their expressions to:
Fixed values (as in the Retract, which is set to '10').
Variable values (as in the Depth, which is set to be the Depth attribute of the hole; not known until the strategy is applied to a particular hole).
In fact you can use a fixed value or any valid equation as an expression.
When used in Decision nodes, expressions are true or false. For example
if Feature.Hole.Diameter=5, then the expression 'Feature.Hole.Diameter<10'
is true, so the decision follows the 
route.
To use an expression:
Activate the box you want to insert it into. To do this you can:
In the Process Browser, double click the value box of a modifier, or right-click on the modifier and select 'Set Modifier'.
In a node you click the Add New Constraint
button.
A rectangle appears round the activated box.
You can now:
Type in the expression; you might use the 'dot' notation as shown in the example above. Do not type spaces into the expression. You can include valid arithmetic and comparison operators.
Double-click on an attribute in the Attribute Browser. The attribute is inserted into the box in dot notation.
Use a combination of the above. You could first double-click on an attribute to insert the first part of the expression (say the 'Hole.Diameter' part), then type in the rest of the equation (say the '<10' part).
After following an 'Enum' type attribute (such as Feature.Hole.Subtype) with an '=', you see a drop down list of the enumerated values (such as HFT_COUNTERBORE, HFT_COUNTERSINK and HFT_DRILLED), you can then select one of these to place on the other side of the '='.
Note that if you make a mistake in the expression; by entering a
string instead of a number for example, you see an error message, preventing
you continuing, until the error is corrected. If you cannot correct the
error, press the Escape key to
abort.
Mistakes in expressions includes mixing data
types.