The actual fuel type limitations will be listed in at least two places: the aircraft type certification data sheet (TCDS), and the limitations section of the aircraft Flight Manual, Operating Handbook, or equivalent.
The fuel placards placed at the fuel filler ports might not list all approved fuels, or may state "Type X fuel only" when additional fuels may in fact be approved. These placards should be understood as a precaution against inadvertently fueling with an unintended or wrong type of fuel. In some cases ignoring these placards can have fatal consequences.
Some aircraft do indeed have approved emergency fuels. This information would also be found in the limitations listed above.
For example, the TCDS for the Beechcraft King Air B200 with PT6A series engines is lists the following for normal approved fuels:
JP-4, JP-5 (MIL-T-5624); JP-8 (MIL-T-83133); JET A, JET A-1, and
JET B conforming to P&WC S.B. 1244 or ASTM SPEC. D1655; in addition for
B200 and B200C Chinese No. 3 Jet Fuel. See NOTE 6 for emergency fuels
Note 6 in the TCDS reads as follows:
NOTE 6. Emergency use of MIL-G-5572:
Grades 80/87, 91/98, 100/130, and 115/145 are permitted for a total time period not to exceed 150 hours time
between engine overhauls. It is not necessary to purge the unused fuel from the system when switching fuel
Likewise, the EASA TCDS for the King Air B200 lists the same limitations.
The Flight Manual for the King Air B200 also lists these limitations, as applicable, as well as some additional altitude constraints when using emergency fuels due to electric boost pump requirements.
In contrast to the actual fuel limitations, the placard placed at the fuel port for the King Air B200 lists very little of the actual limitations:
Source: Beechcraft King Air 200 Maintenance Manual