Both of them refer to the same thing, namely your one and only Employee data source.
"[@identifier]" is global disambiguation syntax supported by the PowerApps Language. Any global entity, such as a control, data source, collection, dynamic data source, etc -- can be referenced by its global disambiguated syntax. This is particularly useful in cases where the name collides with a different identifier, for example the name of a column is a Filter operation:
Filter(HealthRecords, Employee.Age < First([@Employee]).Age)
In the formula above, the first "Employee" refers to a column (field) of HealthRecords, whereas the second disambiguated [@Employee] refers to the global data source. If I used Employee in both cases that would have been a logical (and type) error.
Please note that Intellisense often suggests both forms of an identifier, specificallty to raise awareness about the need to disambiguate in certain scenarios (particularly in row scoping, e.g. Filter, Sort, GroupBy, etc).