BIPM (the folk who define SI)'s 2019 update to the SI Brochure section 5.4.4 says

The symbol used to separate the integral part of a number from its decimal part is called the decimal marker. Following a decision by the 22nd CGPM (2003, Resolution 10), the decimal marker “shall be either the point on the line or the comma on the line.” The decimal marker chosen should be that which is customary in the language and context concerned.

If the number is between +1 and −1, then the decimal marker is always preceded by a zero.

Following the 9th CGPM (1948, Resolution 7) and the 22nd CGPM (2003, Resolution 10), for numbers with many digits the digits may be divided into groups of three by a space, in order to facilitate reading. Neither dots nor commas are inserted in the spaces between groups of three. However, when there are only four digits before or after the decimal marker, it is customary not to use a space to isolate a single digit. The practice of grouping digits in this way is a matter of choice; it is not always followed in certain specialized applications such as engineering drawings, financial statements and scripts to be read by a computer.

For numbers in a table, the format used should not vary within one column.

In CLDR 47, all locales use one of

• Arabic Decimal Separator U+066B (it's comma-shaped)
• ASCII comma, U+002C
• ASCII point, U+002E

as fractional part separator. All of these are on the line and can be described as point or comma on the line. I haven't found any BIPM reference to whether to use localized digits as well as fractional part separators, but using ASCII digits should at least be an option available regardless of locale; when used, if the Arabic separator would otherwise be used, I consider it sensible to use the ASCII comma in place of it, as a sort of converse of LDML's advice (some way into this section):

Locales that specify a numbering system other than "latn" as the default should also specify number formatting symbols that are appropriate for use within the context of the given numbering system. For example, a locale that uses the Arabic-Indic digits as its default would likely use an Arabic comma for the grouping separator rather than the ASCII comma.

CLDR's LDML has (a little below this table) a passing observation:

The grouping separator may also occur in the fractional part, … This is most commonly done where the grouping separator character is a thin, non-breaking space (U+202F), such as “1.618 033 988 75”.

suggesting U+202F as the space character to use for grouping digits. QLocale already accepts plain ASCII space as a substitute for U+202F when parsing numbers. NIST guidance (Item #16, Digit Spacing) supports this choice ("thin fixed space" being a fair description of U+202F):

The digits of numerical values having more than four digits on either side of the decimal marker are separated into groups of three using a thin, fixed space counting from both the left and right of the decimal marker. Commas are not used to separate digits into groups of three.

Authors of scientific works for publication, particularly academics, would likely be glad to have support for BIPM-compatible number formatting. This would naturally be an API of QLocale, design to be decided, taking the above into account.