The text says: "This element specifies a single custom tab stop within a set of custom tab stops applied as part of a set of customized paragraph properties in a document."
This seems to imply that those tabs apply at paragraph level.
However, on page 142, we find in the description of the "pos" attribute: "Specifies the position of the current custom tab stop with respect to the current page margins."
In the case of a RTL paragraph within a LTR page, it is not clear if the pos (tab stop position) value is measured starting from the left (page margins) or from the right (paragraph properties).

The tab position should start from the right margin in RTL paragraphs. Make this clear in the text.

tab (Custom Tab Stop), page 141
Section 2.3.1.37

te

Proposed Disposition of DIS 29500 Comment IL-0028 (Modified: 2007-12-04) Agreed; the following changes will be made to Part 4, §2.3.1.37, page 141, line 15 through page 142, line 2: This element specifies a single custom tab stop within a set of custom tab stops applied as part of a set of customized defined within a set of paragraph properties in a document. A tab stop location shall always be measured relative to the leading edge of the paragraph in which it is used (that is, the left edge for a left-to-right paragraph, and the right edge for a right-to-left paragraph). [Example: Consider a custom tab stop s at 1.5" in a WordprocessingML document. This tab stop would be contained within a tab element defining the tab stop as follows: <w:tab w:val="left" w:pos="2160" /> The tab element specifies all of the properties for the custom ized tab stop for the current paragraph property set. end example]