From BlenderWiki

Limit Location Constraint

Description

An object or unconnected bone can be moved around the scene along the X, Y and Z axes. This constraint restricts the amount of allowed translations along each axis, through lower and upper bounds.

The limits for an object are calculated from its center, and the limits of a bone, from its root.

It is interesting to note that even though the constraint limits the visual and rendered location of its owner, its owner’s data block still allows (by default) the object or bone to have coordinates outside the minimum and maximum ranges. This can be seen in its Transform Properties panel (N). When an owner is grabbed and attempted to be moved outside the limit boundaries, it will be constrained to those boundaries visually and when rendered, but internally, its coordinates will still be changed beyond the limits. If the constraint is removed, its ex-owner will seem to jump to its internally specified location.

Similarly, if its owner has an internal location that is beyond the limits, dragging it back into the limit area will appear to do nothing until the internal coordinates are back within the limit threshold (unless you enabled the For Transform option, see below).

Setting equal the min and max values of an axis, locks the owner’s movement along that axis… Although this is possible, using the Transformation Properties axis locking feature is probably easier!

Options

Target

This constraint uses no target.

minX, minY, minZ

These buttons enable the lower boundary for the location of the owner’s center along respectively the X, Y and Z axes of the chosen CSpace.

The numeric field to their right controls the value of their limit.

Note that if a min value is higher than its corresponding max value, the constraint behaves as if it had the same value as the max one.

maxX, maxY, maxZ

These buttons enable the upper boundary for the location of the owner’s center along respectively the X, Y and Z axes of the chosen CSpace.

The numeric field to their right controls the value of their limit.

For Transform

We saw that by default, even though visually constrained, the owner can still have coordinates out of bounds (as shown by the Transform Properties panel). Well, when you enable this button, this is no more possible – the owner’s transform properties are also limited by the constraint.

Note however that the constraint does not directly modifies the coordinates: you have to grab one way or the other its owner, for this to take effect…

CSpace

This constraint allows you to chose in which space evaluate its owner’s transform properties.

Example

In the examples pictures below, illustrating a Limit Location constraint on the Owner empty object, limiting the X coordinates to the 0.0 value, look, in the Transform Properties panel, at the LocX numeric field value.

Limit Location on an object example.

No constraint.	Owner limited to 0.0 on the X axis.
Grabbing Owner with For Transform disabled.	Grabbing Owner with For Transform enabled.