Generally, fixed allowances provided by an employer are considered as taxable income while work-related expenses are generally not brought to tax.
Let say you went on an outstation trip and incurred expenses on accommodation, petrol and toll. You spent RM1k and company reimburses you the sum. Since you do not earn any extra, so you won’t be taxed on your allowance. However, if your company gives you RM1.2k, you should bring the difference of RM200 to tax because it’s above what you spent already.
Since Budget 2009, there’s an exemptions for travel/petrol allowances:
Up to RM2,400 for home/office travel
Up to RM6,000 for business travel
If you’re given RM1k allowance for you to travel from home to office, then this entire amount will be exempted in full. But, if you’re given RM4k, RM1.6k will be taxable (RM4k – RM2.4k) since exemption is only up to RM2.4k.