T5 lamps usually run on electronic ballasts. Why 4, 6, 8 and 13 watt T5 tubes are commonly used with magnetic ballasts and despite EU energy efficiency regulations, they still come with halophosphate colors? I can not find any information on the Internet... Is there some obscure standard covering these short T5 tubes? Other thing I have wondered is 30 and 15 watt T8/T12 tubes. They seem to have different gas filling and start easily compared to other T8 tubes. Why are just these sizes different? Why datasheets do not tell? Why are these things so obscure?

These are "miniature" fluorescents, the only thing they have in common with the modern T5HE OR T5HO is the tube diameter, nothing else.
The miniature are very old designs, their closest relatives are the T12's.
So the regulators see them as "special use only", with very low energy consumption impact and with the market itself strongly pushing them out, so they do not see the need to intervene.
In the US they fall below practically all thresholds ever imposed (light output, power levels,...), so are "flying under the radars", so again no attention.

Ok. Thanks for information. It is still weird that manufacturers do not tell much about these lamp types. I have old fixture using 8 W T5 and the ballast is also suitable for 4 and 6 watt, but there is no info available what the electrical characteristics of the tubes are. I think it is weird that one ballast can run 3 different wattage lamps. Do someone know what the specifications for these tubes are? Do ballast manufacturers just use trial and error when designing ballasts? There need to be specifications somewhere. Even LEDs seem to be less obscure compared to fluorescents...

I have old fixture using 8 W T5 and the ballast is also suitable for 4 and 6 watt, but there is no info available what the electrical characteristics of the tubes are.

All are of practically the same rated current: 155..170mA (the intention is to have them compatible with a common ballast, where lower wattage, so lower arc voltage leads to a bit higher current).
This current specification was then used with the later nonintegrated CFL (PL-S) of 5..11W, as the wattages were the same range, so using the same ballast was quite convenient (the only exceptions are the US PLS-13W replacing the EUs 11W and then the higher wattages, because the arc voltage would become too high for just a plain series choke in the target market).

So:
F4T5 is 0.17A/28V
F6T5 0.165A/40V
F8T5 0.16A/56V
F13T5 0.15A/96V

The F6T5 is sometimes specified/rated for 0.15A or lower for use in battery operated emergency or camping lanterns, the reason is, it is the type delivering maximum output from input power in the range of 1..3W, the range most of the emergency lanterns operate. The reason for special lamps is the intention to make their lifetime a bit longer by allowing the cathode to heat up a bit more than the standard F6T5, without actually dissipating more power there...
With these the batteries are the way most expensive part, so the design sacrifices the lamp life for the battery runtime.
So instead of using F4T5 at full power (that would require about 5..6W input when counting ballast losses), with F6T5 you are at about 2.5W input (2W into the actual lamp; with the same light output as 5W with F4T5).

Thanks! It is weird that datasheet information is so scarce.

Thanks! It is weird that datasheet information is so scarce.

It is in the corresponding IEC standards, so makers need nothing more than specify the compliance with these standards (as such compliance is usually required by laws/codes for consumer products).
Theoretically you may freely buy these publications, but it is not for free, it costs some bucks.