Ingredient strength is ideally expressed as a ratio of quantities, but there are cases where this is not current practice.  Some expressions are too complex for a simple ratio, generally because they mix in other concepts - but this is the way systems work. And some systems just don't hold ratios as discrete data, but as text.

Example:

Specific Activity 2.3 to 4.2 x 106 U/mg protein

It is proposed to provide a "strength text" field for both the presentation and concentration versions of strength, in addition to structured ones.

Ingredient.strength.presentationText

and 

Ingredient.strength.concentrationText

An option that has been considered but rejected, is to have a choice of structured data OR text. There are cases where legacy data can be partially converted to a structured strength, but with some text left over. If it was a choice of structured or text this would force systems that have to use text to not take advantage of the structure.