Every function needs at least one non-primary modality path

What it is

Every function should have at least one path that does not depend on the primary modality. The primary modality may be unavailable due to environment, device, or user capability, and without a substitute the function becomes unusable in that situation. The requirement is functional reachability rather than channel-level redundancy.

Why it happens

A single-modality path binds function availability to modality availability. Since availability fluctuates—noise, light, social setting, assistive device state—binding means intermittent failure. Offering an alternative keeps the function reachable in any situation, at the cost of maintaining full semantics and state on each path. For it to work, the alternative should reach functional parity; offering only a reduced version denies full function to users who take that path, a quiet form of discrimination.

Studying it

Audit functional reachability: list every function and all its modality paths, check whether the function remains reachable with any single modality disabled, and assess the completeness of each alternative. Variables include function count, number of alternative paths, and completeness. Outcomes include the proportion reachable and the degree of parity, the latter requiring item-by-item comparison with the primary path's capability.

Where it stops holding

Some functions depend on a modality-unique property, such as precise spatial localization; there the alternative may only approximate, and the achievable degree should be documented. If a function's context is strictly limited and the modality is always available, an alternative matters less but should still exist for accessibility. If maintaining an alternative is prohibitively costly, narrowing the function's scope beats shipping an incomplete substitute.

Applying it

• Build a function-to-modality-path map and check individually whether each function is reachable with any modality disabled.
• Ensure alternatives reach functional parity rather than offering a reduced version.
• For functions that cannot be fully substituted, state the achievable degree.
• Verification: with each modality disabled in turn, test reachability and log unreachable items and missing capabilities as a fix list.

Related

• Within the group: D5.04.1 Ambient noise, lighting, and social setting determine modality availability · D5.04.3 When availability changes, suggest the alternative path
• Adjacent: J3.04 Redundancy requirements under multiple impairments · D5.09.3 The switch entry point should be reachable at any time
• Search terms: alternative path · functional parity · accessibility