How it works
Concat appends the second sequence after the first without removing repeated values. Union performs a set-style combination and excludes values that are equal under its comparer. The sample keeps both B and b in the concatenation, but the case-insensitive union treats them as the same identifier.
The union result is explicitly ordered before display. That separates the equality rule used to decide membership from the ordering rule used to present values. A report should not rely on a set operation as though it were an implicit sort, especially when a stable presentation order matters to users or tests.
Neither operator merges the contents of complex records. If two customer rows have the same identifier but different addresses, whole-record Union may keep both, while key-based selection may retain only one representative. A real merge requires a documented conflict-resolution policy; a set operator cannot infer which address is authoritative.
Concat is often correct for event logs where repeated events are meaningful observations. Union can be correct for a list of distinct allowed identifiers. Replacing one with the other because duplicate output looks untidy can erase counts or change the meaning of a calculation. Ask what a repeated value represents in the domain.
Practice by adding A twice to the first sequence and c to the second. Predict both counts and outputs under ordinal and ordinal-ignore-case comparison. Then apply Distinct to the concatenation and explain how that pipeline relates to Union for these simple strings.
Finally, write separate functions named AppendObservations and CombineAllowedIds, even if each implementation is only one LINQ call. Names that express the domain contract help callers avoid choosing an operator based on appearance alone. Test empty inputs, duplicates within one input, and duplicates shared between inputs so information preservation is deliberate.
A working example
using System;
using System.Linq;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Threading;
using System.Threading.Tasks;
using System.Runtime.CompilerServices;
string[] first = { "A", "B" };
string[] second = { "b", "C" };
Console.WriteLine(string.Join(",", first.Concat(second)));
Console.WriteLine(string.Join(",", first.Union(second, StringComparer.OrdinalIgnoreCase)
.OrderBy(value => value, StringComparer.Ordinal)));
Expected output
A,B,b,C A,B,C
Concat keeps duplicates; Union applies an equality policy to remove them.