Snapshot is a flag or value?

[chickendev]

Please help me explain to it, make me confuse a lot.
In a lot of your example, i saw when feed simulator generate new value and call OnEvent(), isSnapshot always true, I understand because it is real-time data,
But in case first client subscribed, feed simulator still generate new value and send to it. In some real project, it get last value from database and set isSnapshot to true.
So I want to ask. snapshot is value ? If it is value, where I can access it ? And why we not send directly snapshot, instead get from 3rd like database. My LS server always in running.
In case 2nd client subscribed same item with first client and request snapshot, MERGE MODE, current itemUpdate of item1 will be sent to client 2 with tag snapshot=true ? Or with distinct mode, current items in itemeventbuffer will be sent with tag ? instead cached value and make it as snapshot value and send ?
And, I saw we can set tag when sent, in case I got old value but set snapshot to false and send, it still valid ? From client or another side, how to check that ?
Thanks for support. <3