Snapshot and RCSI.When does SQL Server go to VersionStore?
Referencing the documentation about Understanding Row Versioning-Based Isolation Levels, I've extracted what I think are the important take-aways with regards to your question, but I'd recommend reading the entire post for additional valuable information.


  When either READ_COMMITTED_SNAPSHOT or ALLOW_SNAPSHOT_ISOLATION
  database option is set ON, the SQL Server Database Engine assigns a
  transaction sequence number (XSN) to each transaction that manipulates
  data using row versioning. Transactions start at the time a BEGIN
  TRANSACTION statement is executed. However, the transaction sequence
  number starts with the first read or write operation after the BEGIN
  TRANSACTION statement. The transaction sequence number is incremented
  by one each time it is assigned.
  
  When either the READ_COMMITTED_SNAPSHOT or
  ALLOW_SNAPSHOT_ISOLATION database options are ON, logical copies
  (versions) are maintained for all data modifications performed in the
  database. Every time a row is modified by a specific transaction, the
  instance of the Database Engine stores a version of the previously
  committed image of the row in tempdb. Each version is marked with
  the transaction sequence number of the transaction that made the
  change. The versions of modified rows are chained using a link list.
  The newest row value is always stored in the current database and
  chained to the versioned rows stored in tempdb.
  
  Row versions are held long enough to satisfy the requirements of
  transactions running under row versioning-based isolation levels. The
  Database Engine tracks the earliest useful transaction sequence number
  and periodically deletes all row versions stamped with transaction
  sequence numbers that are lower than the earliest useful sequence
  number.
  
  Behavior When Reading Data
  
  When a transaction using the snapshot isolation level starts, the
  instance of the Database Engine records all of the currently active
  transactions. When the snapshot transaction reads a row that has a
  version chain, the Database Engine follows the chain and retrieves the
  row where the transaction sequence number is:
  
  
  Closest to but lower than the sequence number of the snapshot    transaction reading the row.
  Not in the list of the transactions active when the snapshot    transaction started.
  
  
  Read operations performed by a snapshot transaction retrieve the last
  version of each row that had been committed at the time the snapshot
  transaction started. This provides a transactionally consistent
  snapshot of the data as it existed at the start of the transaction.
  
  Read-committed transactions using row versioning operate in much the
  same way. The difference is that the read-committed transaction does
  not use its own transaction sequence number when choosing row
  versions. Each time a statement is started, the read-committed
  transaction reads the latest transaction sequence number issued for
  that instance of the Database Engine. This is the transaction sequence
  number used to select the correct row versions for that statement.
  This allows read-committed transactions to see a snapshot of the data
  as it exists at the start of each statement.