CI identifiers can be viewed under the CI record1. A CI identifier is a set of attributes that uniquely identify a CI of a specific class2. CI identifiers are used by the identification and reconciliation engine (IRE) to match incoming data with existing CIs in the CMDB3. CI identifiers can be viewed and edited in the CI record by clicking the Identification Information related link1.
:
1: View and edit CI identifiers
2: Create or edit a CI identification rule
3: Identification rules

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