We've had quite a bit of discussion about this here at Princeton.  I 
think, in the end, it comes down to what you want to do with the data.
In our situation we realized that most of the time, while the 
granularity was very cool, we weren't likely to be doing anything 
special with our deeper (sometimes 3-layers!) mods:relatedItems--it was 
mostly just descriptive information, and we could just as easily 
describe the deeper, more finite relationship in mods:note, 
mods:contents, mods:part, etc.

As a lover of structured data (and the more structure the better), this 
was a tough pill for me to swallow, and I still find myself temped to 
nest mods:relatedItem ad infinitum.  But there were two big tradeoffs 
for us: 1) less potential confusion from those creating the data and 2) 
easier processing.

I'd be curious to hear what others have to say....


Joe Altimus wrote:
> I'm describing videos of dance concerts. Typically, a concert includes 
> several distinct pieces with unique titles, dancers, production 
> personnel and music, so I've used <relatedItem type="constituent"> to 
> represent the details about the individual pieces. Occasionally, one 
> of the individual pieces itself has parts with unique dancers, music, 
> etc. In that case do others consider nesting <relatedItem 
> type="constituent"> the best way to represent that level of detail in 
> the description?
> In skeletal form:
>    <relatedItem type="constituent">
>       ... details about 1st piece in the concert
>    </relatedItem>
>    <relatedItem type="constituent">
>       ... details about 2nd piece in the concert in general, which has 
> its own parts
>          <relatedItem type="constituent">
>             ... details about 1st part of the 2nd piece in the concert
>          </relatedItem>
>          <relatedItem type="constituent">
>             ... details about 2nd part of the 2nd piece in the concert
>          </relatedItem>
>          ... etc.
>    </relatedItem>
>    <relatedItem type="constituent">
>       ... details about 3rd piece in the concert
>    </relatedItem>
>    ... etc.
> Joe Altimus
> Arizona State University Libraries