Blurb::
Metric that estimates active subspace size

Description::
Metric that controls how many basis vectors are retained in the active
subspace.

<b> Default Behavior </b>

The default is to use the \ref
model-active_subspace-truncation_method-constantine diagnostic.

<b> Usage Tips </b>

If the automated subspace identification methods do not yield desirable results,
consider using the explicit \ref model-active_subspace-dimension truncation option.

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