Preserve layer transparency how please

I am using a tutorial created for photoshop4 where a checkbox for switching on/off Preserve Layer Transparency. How is this done in Photoshop7 please, with thanks BB

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I am using a tutorial created for photoshop4 where a checkbox for switching on/off Preserve Layer Transparency. How is this done in Photoshop7 please, with thanks BB

Right above the list of layers is the word "Lock:" with some icons to the right of it. The first one is, I think, equivalent to the "Preserve Layer Transparency" checkbox. It's a square with a grey/white checkerboard pattern, and a tool tip of "Lock transparent pixels."