Ernestkgann, The AIP and Jepp's are only a summary of the regulatory rules, the actual rule document is best used for reference.
You ask where this requirement for inflight alternate airport is?
The rule/regs for an airline operation require... "Each holder of an air operator certificate to establish a fuel policy for both flight planning and inflight requirements..."
Every "air operator" has their approved exposition (manuals) which contains the company fuel policy.
The rule/regs for airline operations, require the inflight fuel to include reserves to cover operation at all times....
To comply with this, the operators exposition's use's the aerodrome alternate requirements or special alternate requirements as a trigger for the inflight fuel policy.
Refer to the rule/reg for... "Air Operations" - for further guidance.
Rgs