Are the following statements valid?

[I just want to check if the following assumptions are generally  
acceptible . If 5 is ok, it makes some arguments easier  ]

A Concept in SKOS represents "a unit of thought [..] combining some or  
all of the characteristics of a concrete or abstract, real or  
imaginary object." (Z39.19, p. 4)
Concepts are represented in textual forms by  indexing terms.    
(Z39.19, p. 6).
indexing is the process of assigning indexing terms to content objects  
(Z39.19 p6).
Content objects are things which convey information/data. (Z39.19, p.  
4).
The extension of a concept is the set of all content objects which  
(are? could be?)  indexed with indexing terms corresponding to that  
concept.