In so many words
Given the clear-cut and unambiguous nature of software, how to explain the plethora of “standard” definitions pertaining to systems, not to mention enterprises, architectures ?

Tentative answers can be found with reference to the core functions documents are meant to support: instrument of governance, medium of exchange, and content storage.
Instrument of Governance: the letter of the law
The primary role of documents is to support the continuity of corporate identity and activities with regard to their regulatory and business environments. Along that perspective documents are to receive legal tender for the definitions of parties (collective or individuals), roles, and contracts. Such documents are meant to support the letter of the law, whether set at government, industry, or corporate level. When set at corporate level that letter may be used to assess the capability and maturity of architectures, organizations, and processes. Whatever the level, and given their role for legal tender or assessment, those documents have to rely on formal textual definitions, possibly supplemented with models.
Medium of Exchange: the spirit of the law
Independently of their formal role, documents are used as medium of exchange, across corporate entities as well as internally between their organizational units. When freed from legal or governance duties, such documents don’t have to carry authorized or frozen interpretations and assorted meanings can be discussed and consolidated in line with the spirit of the law. That makes room for model-based documents standing on their own, with textual definitions possibly set in the background. Given the importance of direct discussions in the interpretation of their contents, documents used as medium of (immediate) exchange should not be confused with those used as means of storage (exchange along time).
Means of Storage: letter only
Whatever their customary functions, documents can be used to store contents to be reinstated at a later stage. In that case, and contrary to direct (aka immediate) exchange, interpretations cannot be consolidated through discussion but have to stand on the letter of the documents themselves. When set by regulatory or organizational processes, canonical interpretations can be retrieved from primary contexts, concerns, or pragmatics. But things can be more problematic when storage is performed for its own purpose, without formal reference context. That can be illustrated by legacy applications with binary code can be accompanied by self-documented source code, source with documentation, source with requirements, generated source with models, etc.
Documentation and Enterprise Architecture
Assuming that the governance of structured social organizations must be supported by comprehensive documentation, documents must be seen as a necessary and intrinsic component of enterprise architectures and their design should be aligned on concerns and capabilities.
As noted above, each of the basic functionalities comes with specific constraints; as a consequence a sound documentation policy should not mix functionalities. On that basis, documents should be defined by mapping purposes with users across enterprise architecture layers:
- With regard to corporate environment, documentation requirements are set by legal constraints, directly (regulations and contracts) or indirectly (customary framework for transactions, traceability and audit).
- With regard to organization, documents have to met two different objectives. As a medium of exchange they are meant to support the collaboration between organizational units, both at business level (processes) and across architecture levels. As an instrument of governance they are used to assess architecture capabilities and processes performances. Documents supporting those objectives are best kept separate if negative side effects are to be avoided.
- With regard to systems functionalities, documents can be introduced for procurements (governance), development (exchange), and change (storage).
- Within systems, the objective is to support operational deployment and maintenance of software components.

The next step will be to integrate documents pertaining to actual environments and organization (brown background) with those targeting symbolic artifacts (blue background).

That could be achieved with MBE/MDA approaches.
Further readings
- Modeling Paradigm
- Enterprise Governance & Knowledge
- Knowledge Architecture
- Caminao & CMMI
- Abstractions & Emerging Architectures
- EA & MDA