Are segments stores on heap or off heap?

Here it says off-heap:

But the stats for this particular shard in my index...

"segments" : {
"count" : 62,
"memory" : "458.7mb",
"memory_in_bytes" : 481075996,
"index_writer_memory" : "0b",
"index_writer_memory_in_bytes" : 0,
"index_writer_max_memory" : "500kb",
"index_writer_max_memory_in_bytes" : 512000,
"version_map_memory" : "0b",
"version_map_memory_in_bytes" : 0,
"fixed_bit_set" : "0b",
"fixed_bit_set_memory_in_bytes" : 0

Is that 458.7mb in on heap memory?

Segments need to store some data into memory in order to be searchable efficiently. This number returns the number of bytes that are used for that purpose. A value of -1 indicates that Elasticsearch was not able to compute this number.