The documentation is currently not up-to-date, you'll have to dig up old discussions in the beta & rc threads (you can look for my posts in the beta thread).
The "chain" mechanism was introduce pretty late (until one of the rcs). Previously in the beta, this was as introduced in the documentation in January:
IP Routing - RouterOS - MikroTik Documentation
(here is the documentation changelog)
Previously, under 7.21 and older, when you added routing rules to the table, the rules were implicitly placed at the location pointed at by the green arrow, between the built-in rule #3 (local address lookup) and #4 (main table fallback). That's why when no routing rules matched, the default action was to lookup in the main table.
The betas of 7.22 made those invisible default rules visible, and you were expected to insert your rules between position #3 and #4 to have the same effects as under older versions.
But soon there were many problems with this approach, especially with export & import, as well as adding new rules (example).
So later in one of the RC versions, the built-in routing rules were removed. To keep the ability for moving custom routing rules as well as the implicit ones around, the new /routing settings policy-rules parameter has been introduced, which now has a list of chain names. This is the default content:
policy-rules=mangle,vrf-lookup,vrf-unreach,local,user,main
As you can see, the user chain is placed by default exactly at the position between local and main, and this corresponds to the position between #3 and #4 mentioned above. To keep the behavior from the previous versions, when you add routing rules without specifying the chain with the command line (not with WinBox, because of the current bug in WinBox), the rules implicitly are in the user chain. NOT local.
Of course, to be able to put custom rules around the built-in implicit rules, placing all rules in the place of user is not enough. That's why you can define custom chain names too, and place those chain names in different location of the policy-rules array. But the default pre-defined chain for custom rules is user.
Unfortunately, the documentation has not been fully updated, MikroTik only updated it to remove the old built-in rules and to list the new policy-rules setting.
Page Comparison - IP Routing (v.59 vs v.60) - RouterOS - MikroTik Documentation
But there is still the paragraph mentioning:
(any user defined rules typically should be added between rule 3 and rule 4)
although there is nothing showing what rule 3 and rule 4 are supposed to be anymore.
EDIT: Documentation has been updated and old mentions of the rules have been removed.