fang Thanks for your interest in learning Ziva VFX!
When creating an attachment (fixed or sliding) the vertices of the source object will be attached to the "root" of the attachment, and the target end will find the closest point on a face. This doesn't make a big difference when using fixed attachments, as both ends are "stuck", but in the case of sliding attachments, the target end is free to slide on the target body, while the source side remains fixed to the source vertices.
The side you choose to slide will depend on your specific use case.
If you want your skin to slide over your muscles, you would choose to make the muscles the source and the skin the target, this will allow the skin to slide freely over all of the muscles. If you were to chose them in the opposite order, the sliding attachments would stick the skin to individual muscles.
I hope you find this helpful!