difference between "Helvetica neue" and "Helvetica neue LT"

[Alastair]

I have a new client who has provided me with an InDesign template (from his creative agency) to create a series of product data sheets from.

My question is about the relationship between the following fonts (or perhaps it's just how Suitcase Fusion & InDesign CS2 handle this): Helvetica Neue and Helvetica Neue LT. I'm running a G5 PowerMac, OS X 10.4.9, all the latest software updates.

If I open the template with no font installed, I get the message from ID that 'the doc uses fonts not available on my system', and lists them like this:

Helvetica Neue (T1) 35 Thin

All instances of the fonts are highlighted in the doc as being missing.

If I dutifully install an Adobe Helvetica Neue Thin PS, I no longer get this message. However, when I get into the doc and go into the paragraph style sheets | basic char. formats, the Font family is shown as

[Helvetica Neue LT]

the Font Style as

[LT 35 Thin].

(The brackets denote a font that is not available as far as I know).

I don't understand why the document now opens without any complaint and yet is showing all fonts as missing in the style sheets palettes. And is this really an issue of fonts missing or perhaps a PC-Mac naming issue?
And finally, if there are fonts missing, how much difference is there between Helvetica Neue and Helvetica Neue LT.

Any help much appreciated....