1. While mapping fields, only similar field types will be listed for selection. Similarly, and adding new fields, fields with similar data types will be created.
   
2. You will have the option of similar field types, when it comes to mapping the fields. However, there are certain exceptions like Picklist, Multi-Select, Auto-Number, Lookup, Formula, User Lookup and Record owner, etc.
   

   Lookup module fields (parent module)	Field type newly created or fields listed for mapping
   Picklist	Single line
   Multi-select	Multi-line (small)
   Auto-number	Single-line
   Lookup	Lookup or single line
   Formula	Field type will be decided based on the return type of the formula field
   User lookup and record owner	User lookup or Single-line

Data population

1. For importing records - During import, the values for field of lookup fields are populated based on the values present in the imported file. Only if the value is empty, it will be populated based on Lookup record.In case the field value is empty then the system will check if any values are present for these fields in the existing lookup records and replace it. For example, while importing deals, if the contact's phone is empty in the field, then the phone number present in the contact record will be captured directly.
   
2. During Lead conversion - When a lead is converted and a contact, account or deal is created, then the values of the related fields will be populated from the mapped fields in the leads module. However, if the fields have empty values then, value form the lookup will be populated.
   
3. Updating records in the lookup module - The FOL field values will be populated only when the record is updated in the lookup field.
   When the related field in the parent module record is updated it doesn't change the value in the field of lookup fields.
   Similarly, when the field of lookup field is updated it doesn't change the value of the mapped field in the parent module.
4. The field of lookup module will not be listed in the Find and merge - During find and merge, only the lookup fields will be listed and based on the lookup value that is updated in the master record, the data in FOL will be populated.
   
5. During mass owner updates, the related data will be populated in all the FOL fields.
   
6. Value in a field of lookup field will be populated if it has both Read-write and Read-only permission. 

Configurations side

1. Unsupported Fields - The field of lookup is not supported for multi-select lookup, file upload, multi-user fields, or image upload fields.
   
2. Parent lookup field is moved to unused fields/deleted - The dependent fields added to the module layout as fields of lookup will also be moved to unused fields. However, the relationship will still exist.
   If the parent lookup field is deleted from the layout, the dependent fields would still be available, but the relation for the field of lookup will not be existing.
3. These fields cannot be marked as "Unique" fields.
   
4. The character limit of the FOL fields is determined based on the parent field - After the field of lookup fields are created, the character limit of those fields cannot be edited and will be dependent on the parent field.
   
5. Fields that are added to a layout via the field of lookup option cannot be updated through the field update option available in automation or process management such as workflows, blueprint, CommandCenter etc. 
   Similarly, if a field is used in one of those field update actions, it won't be listed to map under a field of lookup.
6. System-defined fields can not be a field of lookup field.
   
7. When multi-currency is enabled, the currency value populated in the FOL is based on the record currency and exchange rate present in the current record.
   
8. If the value is changed in the parent field, the value in FOL won't be changed, and if the value in the child is changed, the value in the parent will not be changed, i.e. 2- way sync is not possible.