I'm designing a brand identity guidelines, I've made a change to the "J" letter in the logo, i'm using Museo Sans Rounded and the "J" has a top line that doesn't match with the isotype. There is a page where that change has to be explained but i don't know the technical name of the J top line.
If you're talking about the capitolized J in Museo Sans Rounded, I think you're referring to the Arm of that letter.
Quote from http://www.typographydeconstructed.com/arm/
Definition: The arm of a letter is the horizontal stroke on some characters that does not connect to a stroke or stem at one or both ends. The top of the capital T and the horizontal strokes of the F and E are examples of arms. Additionally, the diagonal upward stroke on a K is its arm. Sometimes arm is used interchangeably with bar or crossbar or cross stroke.
Arm is often also used to describe the mostly horizontal top stroke of C, double-storey a, G, and other glyphs, to include the finial, terminal, spur, or other elements of the stroke.
Also Known As: crossbar, cross stroke