What's the real meaning of randomize=false using cose-bilkent layout - layout

Question

What the real meaning of randomize=false using cose-bilkent layout ?
I can only read such a simple comment to describe this feature in this extension wiki page.
// Whether to enable incremental mode
randomize: true,
I also find another description for randomize in cytoscape.js-expand-collapse wiki page.
// recommended usage: use cose-bilkent layout with randomize: false to preserve mental map upon expand/collapse
So I have learned that randomize: false is to preserve mental map. But what is the real behavior ?
Suppose that I have run a topology graph using cose-bilkent layout, then call the following snippet to append new nodes and edges to the graph with randomize: false. What will be the expected result on incremental mode ? Should the old nodes and edges keep the relative position as so called mental map , while new nodes and edges should be positioned to a resonable coordinate?
var demoId = 10000;
document.getElementById("append").addEventListener("click", function() {
var tid = cy.nodes()[Math.floor(Math.random()*cy.nodes().length)].data().id;
var sid = ++demoId;
var data = {
nodes : [
{data:{"id":"id"+source, "name":"name"+source}}
],
edges :[
{data:{"id":"id"+source+"-"+target, "source":"id"+source, "target":target}}
]
};
cy.add(data);
cy.layout({
name: 'cose-bilkent',
animate: false,
randomize : false,
fit : false
}).run();
});
See my demo. My test result is that all nodes and edges will be relayout. You could watch some nodes and their neighborhood. The neighborhood will be changed after each layout. I can't get a clear mental map. So could the experts explain how incremental layout task effect here.
document.addEventListener("DOMContentLoaded", function() {
var cy = (window.cy = cytoscape({
container: document.getElementById("cy"),
ready: function() {
this.layout({
name: "cose-bilkent",
animationDuration: 1000
}).run();
},
style: [{
selector: "node",
style: {
'content': 'data(id)',
'text-valign': 'center',
'text-halign': 'center',
'background-color': 'blue',
'color':'red',
'width':'10px',
'height':'10px'
}
},
{
selector: ":parent",
style: {
"background-opacity": 0.333
}
},
{
selector: "edge",
style: {
width: 3,
"line-color": "#ad1a66"
}
}
],
elements: [{"group":"nodes","data":{"id":"p_1","parent":"n_72"}},{"group":"nodes","data":{"id":"p_298","parent":"n_72"}},{"group":"nodes","data":{"id":"p_4","parent":"n_72"}},{"group":"nodes","data":{"id":"p_5","parent":"n_72"}},{"group":"nodes","data":{"id":"p_9","parent":"n_72"}},{"group":"nodes","data":{"id":"p_32","parent":"n_72"}},{"group":"nodes","data":{"id":"p_607","parent":"n_72"}},{"group":"nodes","data":{"id":"p_57","parent":"n_72"}},{"group":"nodes","data":{"id":"p_242","parent":"n_72"}},{"group":"nodes","data":{"id":"p_64","parent":"n_72"}},{"group":"nodes","data":{"id":"p_77","parent":"n_72"}},{"group":"nodes","data":{"id":"p_81","parent":"n_72"}},{"group":"nodes","data":{"id":"p_82","parent":"n_72"}},{"group":"nodes","data":{"id":"p_289","parent":"n_72"}},{"group":"nodes","data":{"id":"p_803","parent":"n_72"}},{"group":"nodes","data":{"id":"n_0"}},{"group":"nodes","data":{"id":"n_72"}},{"group":"edges","data":{"source":"n_0","target":"n_0","id":"n_0-n_0"}},{"group":"edges","data":{"source":"n_0","target":"p_81","id":"n_0-p_81"}},{"group":"edges","data":{"source":"n_0","target":"p_4","id":"n_0-p_4"}},{"group":"edges","data":{"source":"n_0","target":"p_298","id":"n_0-p_298"}},{"group":"edges","data":{"source":"n_0","target":"p_803","id":"n_0-p_803"}},{"group":"edges","data":{"source":"n_0","target":"p_607","id":"n_0-p_607"}},{"group":"edges","data":{"source":"n_0","target":"p_1","id":"n_0-p_1"}},{"group":"edges","data":{"source":"n_0","target":"p_289","id":"n_0-p_289"}},{"group":"edges","data":{"source":"n_0","target":"p_57","id":"n_0-p_57"}},{"group":"edges","data":{"source":"n_0","target":"p_82","id":"n_0-p_82"}},{"group":"edges","data":{"source":"n_0","target":"p_32","id":"n_0-p_32"}},{"group":"edges","data":{"source":"p_77","target":"n_0","id":"p_77-n_0"}},{"group":"edges","data":{"source":"p_64","target":"n_0","id":"p_64-n_0"}},{"group":"edges","data":{"source":"p_607","target":"n_0","id":"p_607-n_0"}},{"group":"edges","data":{"source":"p_5","target":"n_0","id":"p_5-n_0"}},{"group":"edges","data":{"source":"p_9","target":"n_0","id":"p_9-n_0"}},{"group":"edges","data":{"source":"p_9","target":"p_803","id":"p_9-p_803"}},{"group":"edges","data":{"source":"p_242","target":"n_0","id":"p_242-n_0"}}],
}));
var demoId = 10000;
document.getElementById("append").addEventListener("click", function() {
var tid = cy.nodes()[Math.floor(Math.random()*cy.nodes().length)].data().id;
var sid = ++demoId;
var data = {
nodes : [
{data:{"id":sid}}
],
edges :[
{data:{"id":sid+"-"+tid, "source":sid, "target":tid}}
]
};
cy.add(data);
cy.layout({
name: 'cose-bilkent',
animate: false,
randomize : false,
fit : true
}).run();
});
});
body {
font-family: helvetica;
font-size: 14px;
}
#cy {
height: 100%;
width: 90%;
position: absolute;
}
h1 {
opacity: 0.5;
font-size: 1em;
}
button {
margin-right: 10px;
}
<script src="https://unpkg.com/cytoscape/dist/cytoscape.min.js"></script>
<!--polyfills are needed for this extension for old browsers like IE -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/core-js/2.5.7/shim.min.js"></script>
<script src="https://unpkg.com/layout-base/layout-base.js"></script>
<script src="https://unpkg.com/cose-base/cose-base.js"></script>
<script src="https://cdn.jsdelivr.net/npm/cytoscape-cose-bilkent#4.1.0/cytoscape-cose-bilkent.min.js"></script>
<body>
<button id="append" class="button">Append</button>
<div id="cy"></div>
</body>