specs: Add span batch hardfork rule

specs/span-batches.md

@@ -9,6 +9,7 @@
 
 - [Introduction](#introduction)
 - [Span batch format](#span-batch-format)
+- [Span batch Hard Fork Rule](#span-batch-hard-fork-rule)
 - [Optimization Strategies](#optimization-strategies)
   - [Truncating information and storing only necessary data](#truncating-information-and-storing-only-necessary-data)
   - [`tx_data_headers` removal from initial specs](#tx_data_headers-removal-from-initial-specs)
@@ -153,6 +154,20 @@ decoding. For example, lets say bad batcher wrote span batch which `block_count
 the explicit limit, not trying to consume data until EOF is reached. We can also safely preallocate memory for decoding
 because we know the upper limit of memory usage.
 
+## Span batch Hard Fork Rule
+
+Span batch hard fork is activated based on timestamp.
+
+Activation Rule: `x != null && x >= upgradeTime && y != null && y >= upgradeTime`
+
+Let `inclusion_block` be the L1 block when the span batch was first fully derived.
+
+`x == span_start.timestamp`, which is the timestamp of first L2 block timestamp derived from span batch.
+
+`y == inclusion_block.timestamp`, which is the timestamp of L1 block when span batch was first fully derived.
+We need this additional check because span batch hard fork is a derivation update, and
+`x` becomes dependent of the hard fork(we must run span batch decoding to find `x`).
+
 ## Optimization Strategies
 
 ### Truncating information and storing only necessary data