A common question with attachments is: 'does the selection order matter'
tl;dr: Not really for fixed. More so for sliding.
The order of selection will determine what is the "source" and "target" of the attachment.
The mesh selected first becomes the "source" and the mesh selected second becomes the "target." For each vertex on the source mesh, the closest point will be looked up on the target mesh, and an attachment proxy is created.
You can select a group of vertices first, then a mesh object second, this will automatically paint the attachment weights to zero on the unselected vertices on the source mesh.
You can manually paint the attachment weights by right-clicking on the source mesh and selecting paint > zAttachment > your_zAttachment. Values between 0 and 1 are valid and multiplied against the zAttachment stiffness attribute. (weight stiffness stiffnessExponent)
Standard work flow for muscles, say for the bicep:
1. select a small group of vertices on the bicep mesh, at the origin (close to the bone), shift select the bone mesh, and run Ziva > Attachment.
2. Select a new group of vertices at the insertion of the muscle, shift select the bone mesh, and run Ziva > Attachment.
While the behaviour of fixed attachments will be similar when the source and target meshes are interchanged, when it comes to sliding attachments, source and destination meshes become more important.
Think about sliding a coffee cup over a table. Our attachment proxy points on the source mesh stay the same. (In this case the vertices on the bottom of the cup)
The attachment proxy points on the target (table) however are free to slide around the surface of that mesh. The attachment attempts to preserve the length of each proxy pair (depending on how stiff it is).
Imagine switching the source and target of the attachment in this example. It wouldn't make sense. If the attachment proxy points on the table stay the same (say a group of vertices in the middle of the table) the cup might tumble a bit depending on the mesh, but clearly the cup won't be free to slide around on the table.
Happy to clarify further if there's more questions on this stuff