Even if at the first reading, your answer seemed to me way away from my question,
but I think your answer is the right one :
"For each vertex either an x,y pair"
"or, for a weighted mesh,
first the number of bones which influence the vertex,
then
for that many bones: bone index, bind position X, bind position Y, weight"
"A mesh is weighted if the number of vertices > number of UVs"
ex : [ 2, 38, 18.17, 41.57, 0.72255, 39, 12.46, 46.05, 0.27745,
number of bones : 2
indice of first bone : 38
bind position X: 18.17
bind position Y: 41.57
weight: 0.72255
indice of second bone: 39
and so on
Thank you for your kind answer.