Relationships between objects and 'hyperlinks' are a key part of CML/TecML but wherever possible they should be added to the document as a specific list rather than hardcoding in hyperlinks with the <A NAME= HREF=> technology. When an object has an explicit (or implicit) address, that can ve used as part of RELATION content. The content allows for: 1:1 links; 1:n links; m:1 links and m:n links. (Example: <RELATION><XVAR>aspirin:C3</XVAR><ARRAY>peaks3:point-25 peaks3:point-29</ARRAY></RELATION> might link the C3 atom of aspirin to two spectral peaks.
It is possible that RELATION can form the basis of a REACTION element, holding references to the reactants and products.
RELATION can hold the whole range of abstract data types (XVAR, XLIST and ARRAY) as additonal content. The use of this is undefined, and probably domain-specific (e.g. it could hold: action for hyperlinks; rendering inforamtion; conditions of transformations such as chemical reactions, and so on.).
The A->HREF mechanism also allows for relationships (1:1 and n:1). It's less powerful than RELATION since it is: hardcoded and difficult to amend; unable to deal with n:m links; and cannot easily be qualified by constraints.
This allows for: