The correct variable syntax to populate a custom column in a horizontal discovery pattern is $cmdb_ci_linux_server.u_custom_column.INSERT. This syntax indicates that the value of the custom column will be inserted into the cmdb_ci_linux_server table for the current CI. The other options are either invalid or incorrect syntaxes for this operation12.
References:
* Set Parameter Value operation
* Pattern variables

The mid.discovery.max_pattern_payload_size property defines the maximum overall size for the returned payload that comes from patterns. This property is set on the MID Server and applies to all patterns that run on that MID Server. If the payload size exceeds this limit, the pattern execution fails and an error message is logged in the ECC queue. The default value of this property is 10 MB1.
References: 1: ServiceNow Docs - MID Server properties for Discovery and Service Mapping 2: ServiceNow Docs - Discovery payload size and processing

CI reclassification is the process of changing the class of a CI in the CMDB based on the discovery data. It can be done manually or automatically, depending on the configuration settings. CI reclassification can result in data loss if the new class has fewer or different attributes than the original class, and the existing attribute values are not preserved or mapped to the new class. This can happen when a CI is switched to a different class hierarchy, such as from a Linux Server to a Windows Server, or when a CI is downgraded to a less specific class, such as from a Database to a Software Instance. Therefore, switching and downgrading have a higher chance of leading to lost data during CI reclassification.
References:
* CI reclassification
* Reclassify CIs
* Configure automatic CI reclassification

Reconciliation rules are used to determine which data sources can update CI attributes in ServiceNow.
Reconciliation rules can be static or dynamic, and they can be applied to specific CI classes, attributes, or discovery sources. Reconciliation rules can also specify the actions to take when there are unmatched values, such as removing, ignoring, or updating them1.
References:
* 1: Reconciliation rules - Product Documentation: San Diego - Now Support Portal